Concurrency is one of those things that are very difficult to unit test. If you are just trying to test that the code inside each thread is doing what it is supposed to test, may be you should just test this code isolated of the context.
If in this example the threads collaborate to reach a result, may be you can test that collaboration without using threads. That would be done by executing all the collaborative parts sequentially.
If you want to test for race conditions and these kind of things, unit testing is not the best way. You will get tests that sometimes fail and sometimes don´t fail.
To summarize, I think that may be your problem is that you are unit testing in a level too high.
Hope this helps
