[BUGFIX] #5189 Private field isTestMode is used in subclass

Change-Id: I382ab5863812a188c9a3a68af896cf328563f39b
Reviewed-on: http://review.typo3.org/34894
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>