[refactor] clarify abc* tests by referencing p2p objects directly

Summary:
Use object returned from add_p2p_connection to refer to p2ps, or use node.p2ps[0] when it needs to be used across many methods. Don't use the p2p property.

For these files, this is best done by removing some unnecessary (and often overcomplicated) helper functions that can be replaced with a single line of code at the callsite.

This is a backport of core#19804 [1b/4]

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10321