test: Avoid race after connect_nodes

Summary:
Wait until the connection is fully established on both sides (verack).

Backport of core#22817

Test Plan:

ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17381