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).
@whatthejeff It's not quite true, it works correctly since 5.3 with non-static methods defined in userspace: http://3v4l.org/uug6K. But I agree that `method_exists` fits better here.