[avalanche] Fix a bug in the preconsensus mining test

Summary: The test expects the quorum proofs to be finalized but it doesn't wait for it. As a result a single round of vote occurs. It has no impact on the test because the latest votes will complete their finalization as a side effect, but better fix the bug.

Test Plan:

./test/functional/test_runner.py abc_mining_avalanche_preconsensus

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18430