[avalanche] Simplify the quorum functional test by unsing the gen_proof facility

Summary:
This makes the code more readable while not changing the behavior.

Depends on D11615.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_quorum

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11616