[avalanche] Fix abc_p2p_compactproofs flakiness

Summary:
The loop waiting for the message count to increase is not reliable because the lock has to be released between the calls, so a message can sneak in between and cause a timeout on the next iteration.

This diff updates the test to use a more simple and robust approach.

Test Plan:

./test/functional/test_runner.py abc_p2p_compactproofs

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12024