[CLEANUP] Clean up $subject, setUp and tearDown

* make setUp and tearDown protected instead of public
* make $subject protected instead of private
* always provide $subject with a default value NULL
* drop the unsets and __tearDown calls from tearDown

Resolves: #63410
Change-Id: Id51cc702c2d558e954f7759f33952b19471eb34c
Reviewed-on: http://review.typo3.org/34883
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>