[TASK] Disable scheduler-tests if EXT:scheduler not loaded

Change-Id: I62ec38fc27330e74c4bc3eed6ded3a2f4a6de550
Resolves: #51115
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/23134
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring