从一个类别单元测试私有方法？

I have a category on NSString class that contains a private helper method. It would be handy if I could use this method in my unit test. However I have difficulties to expose it. When I create a class extension on NSString and declare the method here, the method is not visible in unit test. And it doesn't matter if I create the class extension in a separate header file, or as a part of unit test .m file. class that contai