[chronik] Fix flaky chronik_avalanche test

Summary: Now that the block finalized validation interface is asynchronous, we need to either sync the queue or wait for chronik to update the status or assertions might fail due to the callback still being enqueued.

Test Plan:

./test/functional/test_runner.py chronik_avalanche

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15366