[avalanche] Add coverage for the cooldown feature

Summary: The avalanche cooldown (see the -avacooldown option) is not tested, let's fix that.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_cooldown

Reviewers: #bitcoin_abc, PiRK, bytesofman

Reviewed By: #bitcoin_abc, PiRK, bytesofman

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14548