[TASK] Code cleanup in /frontend/Tests/Unit/ContentObject/

This patch is just groundwork for upcomming "make test xxx notice free"
patches.

Resolves: #85497
Related: #84456
Releases: master
Change-Id: Ib4517cbccadced40e828ded0f76115f00abb5530
Reviewed-on: https://review.typo3.org/57481


Reviewed-by: Robert van Kammen <rvkammen@hotmail.com>
Tested-by: Robert van Kammen <rvkammen@hotmail.com>
Reviewed-by: Jord de Jong <jord.dejong@windinternet.nl>
Tested-by: Jord de Jong <jord.dejong@windinternet.nl>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>