[seeder] Add a test to check outgoing messages in response to VERACK

Summary:
This is in preparation for testing new logic related to using GETHEADERS
messages to check checkpoints. In addition, we'll be able to flesh out
the existing unit tests a bit more.

Depends on D8623

Test Plan:

ninja bitcoin-seeder check-seeder

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8624