Make error messages from ExcludeMimeType validator working in PHP 5.5

- The ```Zend_Validate_File_ExcludeMimeTypeTest``` tests were forgotten in the ```AllTests.php``` so they were never executed.
- When they were run, they were not working properly (IMHO they could never pass in the past)
- Error mesagess of the validator were affected by PHP bug [#63976](https://bugs.php.net/bug.php?id=63976) - the constant value overloading does not work since PHP 5.5, as the ```$_messageTemplates``` array was constructed using constants values from the parent ```Zend_Validate_File_MimeType``` class. This lead to empty error messages. To fix it, message templates must be redefined in the child class.
- Note the test added won't pass in PHP 5.5 without the fix.