[avalanche] Fix a potential race in the quorum functional test

Summary: Wait for the ready_to_poll status to change after disconnecting the nodes to avoid getting an unexpected vote from the node.

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/D16026