[avalanche] Fix a race in the processor tests

Summary: The quorum detection test creates a new processor but doesn't delete the one from the fixture, causing double registration/deletion upon updated block tip and causing a potential use-after-free issue.

Test Plan: Run the test with ASAN 100 times in a loop.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11903