[avalanche] fix timestamp flakiness in abc_p2p_avalanche_remote_proofs.py

Summary:
The remote proofs returned by the getremoteproof RPC contain a "last_update" timestamp. To ensure the comparison between the expected and actual remote proofs does not fail because of a short timing issue, we need to ensure the inbound connection is added after the mocktime is set for the node.

The inbound connections are used for remote proofs since D14961.

Test Plan:

alias testr="~/dev/bitcoin-abc/build/test/functional/test_runner.py"
for i in {0..20}; do  testr abc_p2p_avalanche_remote* ; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14980