[BUGFIX] Tests: Incomplete mocking in ext:scheduler

Use getMock() instead of new() to create the test subject in order to
suppress execution of __construct() of parent AbstractTask.

Change-Id: Id0c297b845db7064b15a859eefa71830ab12074d
Resolves: #56787
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28273
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn