Transformer tests

the testExceptionWhenSettingExistingDirAsTarget trigged a warning in Transformer line 104. That's why I added the suppress character. With the current settings of phpunit the tests won't fail on warnings but it might be better to suppress this. 

Besides that I fixed the execute testcase so it's working correctly.

Rest of this class shall be tested in a integration test. But only a few lines are left.