Check both the function and the method via '@requires function' annotation

Make `@requires function ReflectionMethod::setAccessible` work according to the [documentation](http://phpunit.de/manual/4.1/en/incomplete-and-skipped-tests.html#incomplete-and-skipped-tests.skipping-tests-using-requires).
I thought about `is_callable` as well, but it seems that it doesn't work with the core classes: http://3v4l.org/XATJ5.