[TASK] Fix category collection functional test

Category collection test now extends FunctionalTestCase
class and is green again.

Resolves: #54914
Releases: 6.2
Change-Id: Idc49bcf59b6e2e4ebae5c4f7a232f0b543b6821d
Reviewed-on: https://review.typo3.org/26756
Reviewed-by: Oliver Klee
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs
Reviewed-by: Marcin Sągol
Tested-by: Marcin Sągol
Reviewed-by: Fabien Udriot
Reviewed-by: Francois Suter
Tested-by: Francois Suter