[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