[BUGFIX] Tests: Incomplete mock in ext:scheduler

Do not call __construct of subject.

Change-Id: I7b936efde9220c847b0a6278ea0d5b3606203e17
Resolves: #56894
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28374
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn