[avalanche] Add test for the parking scenario

Summary: Now that we poll all blocks, the parking code is reachable and deserve to be tested.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6731