[TASK] Follow-up to "Add functional tests for generic persistence"

Test directory changed from "Relation" to "Persistence".

Resolves: #55661
Related: #55360
Related: #55167
Releases: 6.2

Change-Id: I422dbfb6bdad4215bdc70dea775917cf3159605f
Reviewed-on: https://review.typo3.org/28297
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny