[avalanche] Fix flaky proof rank comparison in processor_tests

Summary: If two proofs have the same rank, this test breaks because one of the proofs is not marked as accepted like it should be.

Test Plan:

for I in {0..100}; do ninja check-avalanche-processor_tests || break ; done

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17840