test: use explicit p2p objects where available

Summary:
To make the intent of the tests easier to understand, we reference the
p2p connection objects by their explicit names instead of the p2ps array.

This is a backport of core#20022

Test Plan: test/functional/test_runner.py p2p_getdata wallet_resendwallettransactions

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10343