[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