Merge bitcoin/bitcoin#23403: test: Fix segfault in the psbt_wallet_tests/psbt_updater_test

68018e4c3e76f7e5bebf5f90ffd972c7bf01e0a0 test: Avoid excessive locking of `cs_wallet` (Hennadii Stepanov)
7986faf2e09ea85b1d4564ce910f07a4c4de8685 test: Fix segfault in the psbt_wallet_tests/psbt_updater_test (Hennadii Stepanov)

Pull request description:

  The dcd6eeb64adb2b532f5003cbb86ba65b3c08a87b commit (bitcoin/bitcoin#23288) introduced an intermittent failure in the `psbt_wallet_tests/psbt_updater_test` unit test. See bitcoin/bitcoin#23368.

  The test failure can be easily made reproducible with the following patch:
  ```diff
  --- a/src/scheduler.cpp
  +++ b/src/scheduler.cpp
  @@ -57,6 +57,8 @@ void CScheduler::serviceQueue()
               Function f = taskQueue.begin()->second;
               taskQueue.erase(taskQueue.begin());

  +            UninterruptibleSleep(100ms);
  +
               {
                   // Unlock before calling f, so it can reschedule itself or another task
                   // without deadlocking:
  ```

  This PR implements an idea which was mentioned in the [comment](https://github.com/bitcoin/bitcoin/issues/23368#issuecomment-953796339):
  > Yes, as I said before this looks like a race where the wallet is deleted before stopping the scheduler: [#23368 (comment)](https://github.com/bitcoin/bitcoin/issues/23368#issuecomment-952808824)
  >
  > IIRC, the order should be:
  >
  >    * stop scheduler
  >
  >    * delete wallet
  >
  >    * delete scheduler

  The second commit introduces a refactoring with no behavior change.

  Fixes bitcoin/bitcoin#23368.

ACKs for top commit:
  mjdietzx:
    Code review ACK 68018e4c3e76f7e5bebf5f90ffd972c7bf01e0a0

Tree-SHA512: d9103f6252aab807453628159dec243bc543a2595eecaa04ec761dca3c2370085592c55d6f50967d69a4ac6e8b5827eec30dd9b025132c99b0bb9aa5911ad915

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