test: Fix intermittent failure in ChainStateManager tests

Summary:
Before wiping the ChainStateManager, the validationinterface queue must be drained to avoid accessing deleted memory.

This is a backport of core#27348

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14984