[FOLLOWUP][BUGFIX] EXT:form - themeName can not be overwritten by integrator

This patch fixes the broken unit tests.

Change-Id: Ie180a13dfc51c8a3a6fdb63a0a4842b69ff7236a
Resolves: #70368
Releases: master
Reviewed-on: http://review.typo3.org/43896


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Tested-by: Frans Saris <franssaris@gmail.com>