One trick I've used in Scala code is to put the test suite in the same package as the code under test, which gives the tests access to package-private stuff which are normally inaccessible to other code. That probably could work for Java as well. (And in C++, I guess you could declare the test suite as a "friend" of the code under test --- "C++, the language in which friends can access your private members!".)

I feel a bit funny about this, though; it does make the tests explicitly dependent on stuff that, to the rest of the code, is a hidden implementation detail. So, it's probably better to test only the public interface wherever that's feasible...