[TASK] Enable unittests for EXT:documentation

Travis should start including the unittests for EXT:documentation
in its builds as well.

Change-Id: Ie59945964dbf684d3493b0d7581c41b0cde4750f
Resolves: #51114
Releases: 6.2
Reviewed-on: https://review.typo3.org/23132
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring