[test] Assert the exact list of contenders to be polled in abc_p2p_avalanche_contender_voting

Summary:
This is more accurate than checking for the number of polls. This also makes the test capable of handling slowly connecting peers, which could cause a contender poll to finalize before the other contenders are registered (so the poll count will be lower than the expected 12). By checking the set over the whole quorum we remove this constraint entirely.

Depends on D18040.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D18033