Methods in Objective-C are not really private. The error message you are getting is that the compiler can’t verify that the method you are calling exists as it is not declared in the public interface.
The way to get around this is to expose the private methods in a class category, which tells the compiler that the methods exist.
So add something like this to the top of your test case file:
@interface SUTClass (Testing)
- (void)somePrivateMethodInYourClass;
@end
SUTClass is the actual name of the class you are writing tests for.
This will make your private method visible, and you can test it without the compiler warnings.