Merge #18563: test: Fix unregister_all_during_call cleanup

Summary:
13d2a33537a403ac47a989be92109d3214375b6a Fix unregister_all_during_call cleanup (Russell Yanofsky)

Pull request description:

Use `TestingSetup` fixture to fix `unregister_all_during_call` test not calling `UnregisterBackgroundSignalScheduler`, which could trigger an assert in `RegisterBackgroundSignalScheduler` when called in later tests

Failure reported by fanquake https://github.com/bitcoin/bitcoin/pull/18551#issuecomment-610974251

ACKs for top commit:

MarcoFalke:
  ACK 13d2a33537a403ac47a989be92109d3214375b6a if appveyor unit tests pass

Tree-SHA512: d2ec8ff14c54d97903af50031abfac1f38ec1c3aabc90371cfd5b79481fa69d3d77f339bfdf7d2178fd85e83402f72eda7cf4d339e5bbfa7e6e1a68836643b93

Backport of Core PR18563

Depends on D6654

Test Plan: ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6655