[avalanche] Check unknown inv type in cooldown test

Summary:
Make sure unknown inv types cooldown just like known types do.

Depends on D16813

Test Plan:

test_runner.py abc_p2p_avalanche_cooldown

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16814