[tests] Move test_framework Bitcoin primitives into separate module

Summary:
mininode.py wildcard imports all names from messages.py. This is
to avoid having to change all test scripts that import from mininode.py.

This is a move only change

Backport 1/2 of core PR11648 (commit f9cd9b1)

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2338