runtest in tests behave badly when using dune lang 3.0 #4776
Description
On main, when in a test using dune lang 3.0, calling dune runtest from inside the test triggers an unexpected diff with the run.t file itself, as if the sandbox was breached.
Here is a test illustrating the issue:
$ cat >dune-project <<EOF
> (lang dune 2.8)
> EOF
$ cat >dune <<EOF
> (rule
> (alias runtest)
> (action (diff hello.expected hello.output)))
> EOF
$ echo 'foo' > hello.expected
$ echo 'foo' > hello.output
$ dune runtest
$ cat >dune-project <<EOF
> (lang dune 3.0)
> EOF
$ dune runtest
File "run.t", line 1, characters 0-0:
Error: Files _build/default/run.t and _build/default/run.t.corrected differ.
[1]I discovered this while writing tests for the new mds stanza. It sometimes led me to an infinite nesting of sandbox folders:
_build/.sandbox/40844c15a584a31fa740640e0eb87c89/default/test/blackbox-tests/test-cases/mdx-stanza/new-deps-field.t
/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9
/default/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build/.sandbox
/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build
/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default
/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9
/default/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build/.sandbox
/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build
/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default
/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/default/_build/.sandbox/4d7d3316b70f5fc71abfb2d8c509c0d9/
However I am not sure yet what triggers the infinite nesting.
If i add another $ dune runtest at the end of the test it just hangs with 100% cpu.