[avalanche] Move the proof accessor tests to the peer manager tests

Summary:
The peer manager is responsible for the proof management, so this makes sense to test the proof accessors in its test suite.

Depends on D9766.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, PiRK, majcosta

Reviewed By: #bitcoin_abc, PiRK, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9767