Refactor test for existing peer connection into own function

Summary:
This is a backport of core#20187 [2/4]
https://github.com/bitcoin/bitcoin/pull/20187/commits/e8b215a086d91a8774210bb6ce8d1560aaaf0789

Depends on D10694

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10695