[avalanche] Wait for the local winner instead of immediately asserting

Summary: There might be some residual polls left over after the node has finalized the previous set of contenders, so we should wait until the local winner is found before continuing.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17923