[avalanche] Make contender test more comprehensive

Summary:
The current iteration of this test tries to hand pick a manual winner first and then dances around the possibilities that stem from that. A better approach is to test all of the contenders so we can be sure there are no anomalies, and then select a manual winner that we know isn't already a winner.

Depends on D17695

Test Plan:
Run the test a few times to make sure it is robust:

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

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17696