[avalanche] Fix flaky check for finalized proofs in abc_rpc_getavalancheinfo

Summary:
Working off of D11802 so that we actually wait until the proofs are finalized
instead of using a flaky heuristic.

Test Plan:

ninja
for I in {0..20} ; do ./test/functional/test_runner.py abc_rpc_getavalancheinfo ; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11814