[avalanche] Use the appropriate fixture for proof_expiry test

Summary: This is the only peermanager test with 0 cooldown that does not use the fixture.

Test Plan:

ninja check-avalanche

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16604