[TASK] Cover getContentObject related methods by unit tests.

Cover getContentObject related methods of ContentObjectRenderer
by unit tests.

Resolves: #80737
Releases: master, 8.7, 7.6
Change-Id: Ida957cd18c35d10cfc41f954ffdc15e0a04c03a8
Reviewed-on: https://review.typo3.org/52378


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>