[avalanche] Add a facility for building an compact proofs message in tests

Summary: This removes some of the boilerplate. There is no change in behavior.

Test Plan:

./test/functional/test_runner.py abc_p2p_compactproofs.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11680