[avalanche] Simplify the peermanager tests by factoring the buildProof lambdas

Summary: This turns the lambdas into a couple of static functions to reduce the code and have a single maintenance point. There is no change in behavior.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11356