[avalanche] Fix a race in contenders voting test

Summary: There is a race where the chain tip may have started polling but contenders have not yet. It is more robust to answer polls until we see local_winner_cid (and the chain tip will have started polling before that, but we ultimately don't care about this).

Test Plan:
Run a few times:

for I in {0..10}; do ./test/functional/test_runner.py abc_p2p_avalanche_contender_voting || break ; done

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17729