[avalanche] Prevent a potential test failure in abc_p2p_avalanche_remote_proofs

Summary: Upon disconnection of a node, the code to clear the associated remote proof is called asynchronously, we wait for the remote proofs to disapear instead of immediately asserting.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_remote_proofs

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14849