Windows cross-build: unit test `system_tests/run_command` fails and functional tests cannot start `bitcoind`

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Current behaviour

Following `doc/build-windows.md` using WSL + MinGW on Windows, build + install succeed and the GUI runs, but tests fail:

1. The unit test `system_tests/run_command` in `test_bitcoin.exe` fails with an assertion on the expected error message.
2. Functional tests (`wallet_basic.py` and `--coverage`) fail to start `bitcoind` with `Assertion failed: base.is_absolute(), file ./util/fs.cpp, line 36`.

So, building and running the GUI works, but the documented unit and functional tests do not pass on this configuration.


### Expected behaviour

After a successful cross-build using `doc/build-windows.md`, the enabled unit tests (`test_bitcoin`) and basic functional tests (like `wallet_basic.py` and `--coverage`) should run and pass on this supported WSL + MinGW configuration, in line with `src/test/README.md` and `test/functional/README.md`.


### Steps to reproduce

All commands are run inside WSL (Ubuntu). The source tree is **not** under `/mnt/c`, but under the WSL filesystem.

1. Clone and enter the repo:
```
git clone https://github.com/bitcoin/bitcoin.git
cd bitcoin
```

2. Build depends (per `doc/build-windows.md`):
```
gmake -C depends HOST=x86_64-w64-mingw32
```

3. Configure and build (per `doc/build-windows.md`):
```
cmake -B build --toolchain depends/x86_64-w64-mingw32/toolchain.cmake
cmake --build build
```

4. Install to Windows (bitcoind/bitcoin-qt work fine from here):
```
cmake --install build --prefix /mnt/c/workspace/bitcoin
```

5. Build unit tests:
```
cmake --build build --target test_bitcoin
```

6. Run the specific unit test:
```
cd ~/bitcoin/build/bin
./test_bitcoin.exe --run_test=system_tests/run_command --log_level=test_suite
```

7. Run a functional test and coverage (from repo root):
```
cd ~/bitcoin
build/test/functional/test_runner.py wallet_basic.py
build/test/functional/test_runner.py --coverage
```

### Relevant log output

Unit test failure:
```
cd ~/bitcoin/build/bin
./test_bitcoin.exe --run_test=system_tests/run_command --log_level=test_suite
```

Output (trimmed):
```
Entering test module "Bitcoin Core Test Suite"
./wallet/test/walletload_tests.cpp(14): Test suite "walletload_tests" is skipped because disabled
... (many test suites skipped because disabled) ...
./test/testnet4_miner_tests.cpp(30): Test suite "testnet4_miner_tests" is skipped because disabled
./test/system_tests.cpp(17): Entering test suite "system_tests"
./test/system_tests.cpp(21): Entering test case "run_command"
./test/system_tests.cpp(56): error: in "system_tests/run_command": check what.find(tfm::format("RunCommandParseJSON error: process(%s) returned 1: \n", command)) != std::string::npos has failed
./test/system_tests.cpp(21): Leaving test case "run_command"; testing time: 196716us
./test/system_tests.cpp(17): Leaving test suite "system_tests"; testing time: 199262us
./test/system_ram_tests.cpp(13): Test suite "system_ram_tests" is skipped because disabled
... (more suites skipped) ...
Leaving test module "Bitcoin Core Test Suite"; testing time: 433839us

*** 1 failure is detected in the test module "Bitcoin Core Test Suite"
```

Functional test failure (wallet_basic.py):
```
cd ~/bitcoin
build/test/functional/test_runner.py wallet_basic.py
```

Output:
```
Temporary test directory at /tmp/test_runner_₿_🏃_20251225_162435
Remaining jobs: [wallet_basic.py]
1/1 - wallet_basic.py failed, Duration: 57 s

stdout:
2025-12-25T16:24:35.518658Z TestFramework (INFO): PRNG seed is: 6050933089769083783
2025-12-25T16:24:35.520147Z TestFramework (INFO): Initializing test directory /tmp/test_runner_₿_🏃_20251225_162435/wallet_basic_0
2025-12-25T16:25:32.647545Z TestFramework (ERROR): Unexpected exception
Traceback (most recent call last):
  File "/home/kelos/bitcoin/test/functional/test_framework/test_framework.py", line 138, in main
    self.setup()
  File "/home/kelos/bitcoin/test/functional/test_framework/test_framework.py", line 269, in setup
    self.setup_network()
  File "/home/kelos/bitcoin/build/test/functional/wallet_basic.py", line 44, in setup_network
    self.setup_nodes()
  File "/home/kelos/bitcoin/test/functional/test_framework/test_framework.py", line 382, in setup_nodes
    self.start_nodes()
  File "/home/kelos/bitcoin/test/functional/test_framework/test_framework.py", line 528, in start_nodes
    node.wait_for_rpc_connection()
  File "/home/kelos/bitcoin/test/functional/test_framework/test_node.py", line 316, in wait_for_rpc_connection
    raise FailedToStartError(self._node_msg(
test_framework.test_node.FailedToStartError: [node 0] bitcoind exited with status 3 during initialization. Assertion failed: base.is_absolute(), file ./util/fs.cpp, line 36
************************

2025-12-25T16:25:32.699370Z TestFramework (INFO): Not stopping nodes as test failed. The dangling processes will be cleaned up later.
2025-12-25T16:25:32.699638Z TestFramework (WARNING): Not cleaning up dir /tmp/test_runner_₿_🏃_20251225_162435/wallet_basic_0
2025-12-25T16:25:32.699739Z TestFramework (ERROR): Test failed. Test logging available at /tmp/test_runner_₿_🏃_20251225_162435/wallet_basic_0/test_framework.log
2025-12-25T16:25:32.699905Z TestFramework (ERROR):
2025-12-25T16:25:32.700105Z TestFramework (ERROR): Hint: Call /home/kelos/bitcoin/test/functional/combine_logs.py '/tmp/test_runner_₿_🏃_20251225_162435/wallet_basic_0' to consolidate all logs
2025-12-25T16:25:32.700184Z TestFramework (ERROR):
2025-12-25T16:25:32.700424Z TestFramework (ERROR): If this failure happened unexpectedly or intermittently, please file a bug and provide a link or upload of the combined log.
2025-12-25T16:25:32.700571Z TestFramework (ERROR): https://github.com/bitcoin/bitcoin/issues
2025-12-25T16:25:32.700677Z TestFramework (ERROR):

stderr:
[node 3] Cleaning up leftover process
[node 2] Cleaning up leftover process
[node 1] Cleaning up leftover process
[node 0] Cleaning up leftover process

TEST            | STATUS    | DURATION
wallet_basic.py | ✖ Failed  | 57 s

ALL             | ✖ Failed  | 57 s (accumulated)
Runtime: 57 s
```

Functional test failure with --coverage:
```
cd ~/bitcoin
build/test/functional/test_runner.py --coverage
```

Output:
```
Temporary test directory at /tmp/test_runner_₿_🏃_20251225_162532
Initializing coverage directory at /tmp/coveragemf6gv9tr
[node 0] Cleaning up leftover process
2025-12-25T16:25:32.916746Z TestFramework (INFO): PRNG seed is: 332623413216244638
2025-12-25T16:25:32.917531Z TestFramework (INFO): Initializing test directory /tmp/test_runner_₿_🏃_20251225_162532/cache
2025-12-25T16:25:35.676350Z TestFramework (ERROR): Unexpected exception
Traceback (most recent call last):
  File "/home/kelos/bitcoin/test/functional/test_framework/test_framework.py", line 138, in main
    self.setup()
  File "/home/kelos/bitcoin/test/functional/test_framework/test_framework.py", line 268, in setup
    self.setup_chain()
  File "/home/kelos/bitcoin/test/functional/test_framework/test_framework.py", line 356, in setup_chain
    self._initialize_chain()
  File "/home/kelos/bitcoin/test/functional/test_framework/test_framework.py", line 814, in _initialize_chain
    self.start_node(CACHE_NODE_ID)
  File "/home/kelos/bitcoin/test/functional/test_framework/test_framework.py", line 514, in start_node
    node.wait_for_rpc_connection()
  File "/home/kelos/bitcoin/test/functional/test_node.py", line 316, in wait_for_rpc_connection
    raise FailedToStartError(self._node_msg(
test_framework.test_node.FailedToStartError: [node 0] bitcoind exited with status 3 during initialization. Assertion failed: base.is_absolute(), file ./util/fs.cpp, line 36
************************

2025-12-25T16:25:35.729866Z TestFramework (INFO): Not stopping nodes as test failed. The dangling processes will be cleaned up later.
2025-12-25T16:25:35.730219Z TestFramework (WARNING): Not cleaning up dir /tmp/test_runner_₿_🏃_20251225_162532/cache
2025-12-25T16:25:35.730333Z TestFramework (ERROR): Test failed. Test logging available at /tmp/test_runner_₿_🏃_20251225_162532/cache/test_framework.log
2025-12-25T16:25:35.730631Z TestFramework (ERROR):
2025-12-25T16:25:35.730888Z TestFramework (ERROR): Hint: Call /home/kelos/bitcoin/test/functional/combine_logs.py '/tmp/test_runner_₿_🏃_20251225_162532/cache' to consolidate all logs
2025-12-25T16:25:35.731025Z TestFramework (ERROR):
2025-12-25T16:25:35.731110Z TestFramework (ERROR): If this failure happened unexpectedly or intermittently, please file a bug and provide a link or upload of the combined log.
2025-12-25T16:25:35.731245Z TestFramework (ERROR): https://github.com/bitcoin/bitcoin/issues
2025-12-25T16:25:35.731393Z TestFramework (ERROR):

Traceback (most recent call last):
  File "/home/kelos/bitcoin/build/test/functional/test_runner.py", line 931, in <module>
    main()
  File "/home/kelos/bitcoin/build/test/functional/test_runner.py", line 542, in main
    run_tests(
  File "/home/kelos/bitcoin/build/test/functional/test_runner.py", line 595, in run_tests
    subprocess.check_output([sys.executable, tests_dir + 'create_cache.py'] + flags + ["--tmpdir=%s/cache" % tmpdir])
  File "/usr/lib/python3.12/subprocess.py", line 466, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.12/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/usr/bin/python3', '/home/kelos/bitcoin/build/test/functional/create_cache.py', '--cachedir=/home/kelos/bitcoin/build/test/cache', '--configfile=/home/kelos/bitcoin/build/test/functional/../config.ini', '--coveragedir=/tmp/coveragemf6gv9tr', '--tmpdir=/tmp/test_runner_₿_🏃_20251225_162532/cache']' returned non-zero exit status 1.
```

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

master@d861c3820528fad2e17a455949bec70b6ba434dcc

### Operating system and version

Windows 10.0.26200.7462 (host), WSL2 with Ubuntu; `wsl.exe --version`: - WSL version: 2.6.2.0 - Kernel version: 6.6.87.2-1 - WSLg version: 1.0.71 - MSRDC version: 1.2.6353 - Direct3D version: 1.611.1-81528511 - DXCore version: 10.0.26100.1-240331-1435.ge-release

### Machine specifications

CPU:
- 12th Gen Intel Core i5-12500H (x86_64)
- 8 cores / 16 threads (lscpu)
- L3 cache: 18 MiB

Memory:
- 16 GiB RAM total
- ~11 GiB free, 4 GiB swap (free -h)

Disk:
- Host C: drive 454 GiB total, 219 GiB free (df -h, /mnt/c)
- WSL root filesystem on /dev/sdd: 1.0 TiB total, 946 GiB free

Distro inside WSL:
- Ubuntu 24.04.3 LTS (noble) (lsb_release -a)

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Windows cross-build: unit test `system_tests/run_command` fails and functional tests cannot start `bitcoind` #34150

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Windows cross-build: unit test system_tests/run_command fails and functional tests cannot start bitcoind #34150

Description

Is there an existing issue for this?

Current behaviour

Expected behaviour

Steps to reproduce

Relevant log output

How did you obtain Bitcoin Core

What version of Bitcoin Core are you using?

Operating system and version

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Windows cross-build: unit test `system_tests/run_command` fails and functional tests cannot start `bitcoind` #34150