[TASK] Add tests for ContentObjectRenderer::getData()

* Adds unittests

Change-Id: I2b4ff89c828384c852e6fbf9f82c55e6ebbac281
Resolves: #50258
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/22641
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind