[avalanche] Extract the facilities to the test object to simplify the abc_p2p_avalanche_proof_voting test

Summary: This is a step toward splitting the test into pieces so we don't have the various cases scattered over files with redundant content.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_proof_voting

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11043