Fix unregister_all_during_call cleanup
Fix unregister_all_during_call cleanup

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 <fanquake@gmail.com>
https://github.com/bitcoin/bitcoin/pull/18551#issuecomment-610974251

Former-commit-id: cabbdc3fc7ec83405a3093f15f34daf150da6e22
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin