[#17] Reviewed File\ExistsTest
[#17] Reviewed File\ExistsTest

- Renamed new test methods to indicate behavior tested.
- Added data provider for testAddDirectoryShouldRaiseExceptionForInvalidArgument
  to ensure all possible invalid types are tested.
- CS fixes per phpcs.

zf2timo/zend-validator,akrabat/zend-validator,svycka/zend-validator,ezimuel/zend-validator,zendframework/zend-validator