test: improve robustness of connect_nodes()

Decoupled from #27837 because this can help other too, found it investigating a CI failure https://cirrus-ci.com/task/5805115213348864?logs=ci#L3200.

The connect_nodes function in the test framework relies on a stable number of peer connections to verify that the new connection between the nodes is successfully established. This approach is fragile, as any of the peers involved in the process can drop, lose, or create a connection at any step, causing subsequent wait_until checks to stall indefinitely even when the peers in question were connected successfully.

This commit improves the situation by using the nodes' subversion and the connection direction (inbound/outbound) to identify the exact peer connection and perform the checks exclusively on it.