[avalanche] Set all local stake winners as accepted contenders

Summary: This is more robust than just accepting the first local winner, especially in the event the network becomes poorly connected.

Test Plan:
Make sure the test is not flaky:

for I in {1..30}; do ninja check-avalanche-processor_tests || break ; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17503