[TASK] Change $fixture to $subject in unit-tests

Subject is the "system under test, the
subject you're operating on"

Resolves: #64771
Releases: master
Change-Id: I7a70d5f44e721b958d86aea741534ace2f53b2a8
Reviewed-on: http://review.typo3.org/36615


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>