DEV: resets user search cache between tests (PR #13873)

The current behaviour was producing random tests failures which where consistently reproducible using seed=32037592518471299633729129648744282271

The cause of this error, is a previous test not giving any topicId or categoryId resulting in a cache key “undefined-undefined”, just like a possibly previous test. Reseting cache between tests, seems the most straightforward and future proof solution