[BUGFIX] Fix failing unit test

A unit test causes a fatal error due to newly introduced dependency
to SignalSlotDispatcher. The fix extends the mocking to prevent
the FATAL error.

Change-Id: I02c6cf2cda07bd965a3cbbd1512d7e80fac8f908
Resolves: #63882
Relates: #62960
Releases: master
Reviewed-on: http://review.typo3.org/35395
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>