[TASK] Add unit tests for forceAbsoluteUrl()

ContentObjectRenderer::forceAbsoluteUrl() is not covered by unit tests
at all.
Add unit tests and improve mocking in other existing tests.

Resolves: #60629
Releases: 6.3, 6.2
Change-Id: Ie3c03b0cff2118bf7ef38ca03135f2a0a6f120cd
Reviewed-on: http://review.typo3.org/31874
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>