Summary:
D31719 attempted to fix the flaky test `TestYbAsh#testEmptyCircularBuffer`.
However, since then multiple background tasks have also been instrumented
which might show up in the buffer even when no queries have been fired.

This diff fixes the test by excluding all background tasks.

Jira: DB-12215

Test Plan: Jenkins: test regex: testEmptyCircularBuffer

Reviewers: jason

Reviewed By: jason

Differential Revision: https://phorge.dev.yugabyte.com/D36885