[avalanche] Add test coverage to ensure immature proofs are handled correctly as contenders

Summary: This builds on D17867 by adding test cases for immature proofs.

Test Plan:

./test/functional/test_runner.py abc_p2p_avalanche_contender_voting

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17879