494455f test: use MiniWallet for feature_fee_estimation.py (Sebastian Falbesoner)

Pull request description:

  This PR enables one more of the non-wallet functional tests (feature_fee_estimation.py) to be run even with the Bitcoin Core wallet disabled by using the MiniWallet instead, as proposed in bitcoin#20078. It takes use of the recently introduced methods `{create,send}_self_transfer_multi` (bitcoin#24637) which allows to specify multiple UTXOs to be spent rather than only one. Very likely the test can still be simplified (e.g. coin selection in `small_txpuzzle_randfee`), but this is a first step.

ACKs for top commit:
  ayush933:
    tACK 494455f . The test runs successfully with the wallet disabled.
  vincenzopalazzo:
    tACK bitcoin@494455f

Tree-SHA512: 89789fc34a4374c79c4b90acd926ac69153aad655dab50450ed796f03c770bd675ad872e906f516f90e8d4cb40b83b55f3c78a94b13bfb8fe8f5e27624937748

…s (feature_fee_estimation.py performance fix)

a498acc test: MiniWallet: skip mempool check if `mempool_valid=False` (Sebastian Falbesoner)
01552e8 test: MiniWallet: always rehash after signing (P2PK mode) (Sebastian Falbesoner)

Pull request description:

  MiniWallet's core method for creating txs (`create_self_transfer`) right now always executes the `testmempoolaccept` RPC to check for mempool validity or invalidity. In some test cases where we use MiniWallet to create a huge number of transactions this can lead to performance issues, in particular feature_fee_estimation.py where the execution time after MiniWallet usage (PR bitcoin#24817) doubled, see bitcoin#24828 (comment), bitcoin#24828 (comment). This PR mitigates this by skipping the mempool check if the parameter `mempool_valid` is set to `False`.

  As a preparatory commit, the test feature_csv_activation.py has to be adapted w.r.t. to rehashing of transactions, as we now hash all transactions immediately in `create_self_transfer` in order to get the txid (before we relied on the result of `testmempoolaccept`).

  On my machine, this decreases the execution time quite noticably:

  master branch:
  ```
  $ time ./test/functional/feature_fee_estimation.py
  real    3m20.771s
  user    2m52.360s
  sys     0m39.340s
  ```

  PR branch:
  ```
  $ time ./test/functional/feature_fee_estimation.py
  real    2m1.386s
  user    1m42.510s
  sys     0m22.980s
  ```

  Partly fixes bitcoin#24828 (hopefully).

ACKs for top commit:
  danielabrozzoni:
    tACK a498acc

Tree-SHA512: f20c358ba42b2ded86175f46ff3ff9eaefb84175cbd1c2624f44904c8d8888e67ce64d6dcbb26aabbf07906e6f5bdea40353eba9ae668618cadcfc517ef7201b

…s (feature_fee_estimation.py performance fix)

a498acc test: MiniWallet: skip mempool check if `mempool_valid=False` (Sebastian Falbesoner)
01552e8 test: MiniWallet: always rehash after signing (P2PK mode) (Sebastian Falbesoner)

Pull request description:

  MiniWallet's core method for creating txs (`create_self_transfer`) right now always executes the `testmempoolaccept` RPC to check for mempool validity or invalidity. In some test cases where we use MiniWallet to create a huge number of transactions this can lead to performance issues, in particular feature_fee_estimation.py where the execution time after MiniWallet usage (PR bitcoin#24817) doubled, see bitcoin#24828 (comment), bitcoin#24828 (comment). This PR mitigates this by skipping the mempool check if the parameter `mempool_valid` is set to `False`.

  As a preparatory commit, the test feature_csv_activation.py has to be adapted w.r.t. to rehashing of transactions, as we now hash all transactions immediately in `create_self_transfer` in order to get the txid (before we relied on the result of `testmempoolaccept`).

  On my machine, this decreases the execution time quite noticably:

  master branch:
  ```
  $ time ./test/functional/feature_fee_estimation.py
  real    3m20.771s
  user    2m52.360s
  sys     0m39.340s
  ```

  PR branch:
  ```
  $ time ./test/functional/feature_fee_estimation.py
  real    2m1.386s
  user    1m42.510s
  sys     0m22.980s
  ```

  Partly fixes bitcoin#24828 (hopefully).

ACKs for top commit:
  danielabrozzoni:
    tACK a498acc

Tree-SHA512: f20c358ba42b2ded86175f46ff3ff9eaefb84175cbd1c2624f44904c8d8888e67ce64d6dcbb26aabbf07906e6f5bdea40353eba9ae668618cadcfc517ef7201b