Well, unit testing should test units and ideally every class is a self-contained unit – this follows directly from the single responsibility principle.
So testing private members of a class shouldn’t be necessary – the class is a black box that can be covered in a unit test as-is.
On the other hand, this isn’t always true, and sometimes with good reasons (for instance, several methods of the class could rely on a private utility function that should be tested). One very simple, very crufty but ultimately successful solution is to put the following into your unit-test file, before including the header that defines your class:
#define private public
Of course, this destroys encapsulation and is evil. But for testing, it serves the purpose.