[backport#18247] test: Wait for both veracks in add_p2p_connection

Summary:
Partial backport of Core PR18247

Depends on D6431

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, jasonbcox, deadalnix

Reviewed By: #bitcoin_abc, jasonbcox, deadalnix

Subscribers: deadalnix, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D6384