[avalanche] Extract the compact proofs functional test to its own file
This is not really related to the proof inventories, and more tests are coming so better split it out. There is no change in behavior.
Depends on D11388.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D11533