[BUGFIX] Fix namespace of RelationTest

Resolves: #58450
Releases: 6.2
Change-Id: I78d39b0be0d4fc9a425ff3fa1e2424ee017d3180
Reviewed-on: https://review.typo3.org/29798
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring