[avalanche] Make the proof builders build a ProofRef

Summary:
This removes some boilerplate in the code, especially in the tests.
Updated the proof builder and test builder failities to keep the code consistent.

Ref T1854.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1854

Differential Revision: https://reviews.bitcoinabc.org/D10389