[avalanche] Refactor proof rank calculation in functional test

Summary: Consolidate the proof reward rank logic into one reusable function.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17993