fa1e69e qa: Sync with validationinterface queue in sync_mempools (MarcoFalke)
Pull request description:
Commit e545dedf72bff2bd41c93c93eb576929fce37112 moved `TransactionAddedToMempool` to the background scheduler thread. Thus, adding a transaction to the mempool will no longer add it to the wallet immediately. Functional tests, that `sync_mempools` and then call into wallet rpcs will race against the scheduler thread.
Fix that race by flushing the scheduler queue.
Fixes#12205; Fixes#12171;
References #9584;
Tree-SHA512: 14d99cff9c4756de9fad412f04e6d8e25bb9a0938f24ed8348de79df5b4ee67763dac5214b1a69e77e60787d81ee642976d1482b1b5637edfc4892a238ed22af
898f560b55