fix tests for newer PHPUnit. fixes #1643

getMock is deprecated in new PHPUnit versions and createMock is the
recommended replacement. However that one is not available in older
PHPUnit releases. Since we still support older PHP releases we have to
support older PHPUnit releases as well.

This add some compatibility functions to our TestCase class and replaces
all calls to getMock. Tested with PHPUnit 4.6.6 and 5.5.0.