[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