test: Move test framework specific constants from messages.py to p2p.py

Summary:

The messages.py module should contain code and helpers for
[de]serializing p2p messages. Specific usage of those messages should
be in p2p.py. This PR moves test framework specific constants to p2p.py.

This is a backport of core#20524
Depends on D10943

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10944