improve code coverage

improve code coverage
This is the current way of fixer works by design, not by a bug, see
https://github.com/FriendsOfPHP/PHP-CS-Fixer/blob/master/Symfony/CS/Fixer/Symfony/UnusedUseFixer.php#L181
So the test tests if the expected behavior is provided.

To support multiple namespaces in this fixer we need some extra work, some of it was already done here: #622, but there are not much intrest in that