[test] Don't assume the proof score in abc_p2p_avalanche_contender_voting

Summary:
The proof score is assumed to be 5000, aka 50M XEC of staked amount. This is only true for as long as no staking reward output is added to the coinbase. This diff removes this assumption for the get_all_contender_ids() function and prevent using the other methods that rely on this fact after it turns out wrong.

This is a step towards fixing the test flakiness.

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/D18037