Fix a race in abc_p2p_avalanche_remote_proofs.py

Summary: When add_outbound_p2p_connection returns there is no guarantee that the node is available from the RPC yet. This diff waits for the node to account for the new remote to fix this.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_remote_proofs

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17387