[avalanche] Consistently sync the validation interface queue before destroying the processor in tests

Summary:
Some tests already make use of SyncWithValidationInterface to avoid the notification handler from running when the Processor is destroyed. Make this consistent across the whole test.

Note the sync at the end of block_reconcile_initial_vote is redundant (and removed), the ~WithAvalanche destructors already syncs so ends of BOOST_AUTO_TEST_CASE are not affected.

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D20158