[BUGFIX] Class 'TYPO3\CMS\Scheduler\Task\AbstractTask' not found

When running UnitTests for UpdateExtensionListTask and the extension
Scheduler is not loaded will raise an Fatal error because it tries to
mock TYPO3\CMS\Extensionmanager\Task\UpdateExtensionListTask which
extends from TYPO3\CMS\Scheduler\Task\AbstractTask.

It is necessary to skip the whole test case because every single test
creates a new instance of
\TYPO3\CMS\Extensionmanager\Task\UpdateExtensionListTask().

Releases: 6.2
Resolves: #55192
Change-Id: I81e69c57d58078cf00e4f8e47a1e8d5c7d984177
Reviewed-on: https://review.typo3.org/26954
Reviewed-by: Wouter Wolters
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer