[avalanche] Processor event loop: make sure we test what we expect

Summary: There is a missing check in the vote registration; if it fails that will make the following query time check totally irrelevant.

Test Plan:

ninja check-avalanche-processor_tests

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13009