[TASK] Tests for Persistence\Generic\Backend::getIdentifierByObject

Change-Id: Idb00a92469672e907286024ec461f0b8ee98c6c9
Resolves: #52570
Releases: 6.1, 6.2
Reviewed-on: https://review.typo3.org/24440
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn