[avalanche] Fix the avalanche test on slow machines

Summary:
Wait for the proof to reach the proof pool.

Ref T1611.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9653