test: Wait until is_connected in add_p2p_connection

Summary:
Moving the wait_until from the individual test scripts to the test framework simplifies two tests

This is a backport of core#19657

Depends on D10064

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10063