[avalanche] Fix threading issue in processor_tests.cpp

Summary:
The test called the PeerManager directly instead of using the processor
interface that holds the appropriated locks.

Test Plan:
With Clang and Debug:

ninja check-avalanche

Run the TSAN build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Maniphest Tasks: T1611

Differential Revision: https://reviews.bitcoinabc.org/D9634