[BUGFIX] Make abstract test class really abstract to exclude it from test runs

Added the missing keyword abstract to the class definition to prevent
phpunit from issuing a warning about the class not containing any tests.

Resolves: #90357
Releases: master, 9.5
Change-Id: I4b2d5c8111d63e73039658efffe73888b0c84179
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63204


Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>