[avalanche] Add test coverage for parking against avalanche finalized chaintip

Summary:
Ensure we do not introduce regressions related to parking blocks near the chaintip.
With avalanche now enabled by default and validation changes planned, testing this
behavior explicitly is very important.

Test Plan:

ninja
./test/functional/test_runner.py abc_p2p_avalanche_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12943