[avalanche] Fix flakiness where finalization of the newly mature proof was not checked along with the other finalized proofs

Summary:
The immature proof is not accounted for in the finalization checks at the end of the test. This makes the finalized_proof_count flaky since all proofs are being voted for.

This patch ensures the immature proof is also finalized.

Test Plan:

ninja
./test/functional/test_runner.py abc_rpc_getavalancheinfo

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11870