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 think its best to use like it already is. Beeing able to also reference non-callable but existing method makes sense in some cases.

Thanks for clarification @whatthejeff