[test] Only activate the quorum after all the peers are connected

Summary:
This ensures the proof gets finalized at the same time, avoiding flakiness.

This is a step towards fixint the test flakiness.

Depends on D18039.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D18040