[avalanche] Fix missing wait_until in contender test

Summary: This check should keep answering polls until the chain tip is found. This just happens to be the case most of the time.

Test Plan:

ninja && ./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17692