[refactor] clarify abc* tests by referencing p2p objects directly
Use object returned from add_p2p_connection to refer to p2ps, or use node.p2ps 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
Differential Revision: https://reviews.bitcoinabc.org/D10321