[avalanche] Fix race condition in processor destructor test

Summary: Example flakiness: https://build.bitcoinabc.org/viewLog.html?buildId=64245&buildTypeId=BitcoinABC_Master_BitcoinAbcMasterAsan&tab=buildLog&_focus=3164

Test Plan:

for i in {0..200} ; do ninja check-avalanche-processor_tests ; done

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7554