[BUGFIX] #5103 TestingFramework::cleanUp needs to discard stored e-mails

Change-Id: I029e29087324ce6c9d4dc71042372c0193f2c0f5
Reviewed-on: http://review.typo3.org/32486
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>