test: Fix segfault in the psbt_wallet_tests/psbt_updater_test #23403

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:

--- 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:

Yes, as I said before this looks like a race where the wallet is deleted before stopping the scheduler: #23368 (comment)

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.

Code review ACK 68018e4c3e76f7e5bebf5f90ffd972c7bf01e0a0