[avalanche] Merge the avalanche voting tests

Summary:
Due to parallel development, 2 functional tests have been written: abc_p2p_avalanche_proof_voting.py and abc_p2p_avalanche_voting_proof.py, which is unfortunate. This diff merge the tests into a single one.

Depends on D11044.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_proof_voting

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11048