In preparation for a talk I'm giving at DOSUG I'm going to post my thoughts as they develop.
Miranda has many Junit tests. 316 at last count. This does not guarantee that there are no bugs, but it does make it more likely.
A typical Miranda test checks that a Message gets to the method expected and is processed as expected.
I have also been pleasantly surprised by how smoothly Mockito has worked. With it, I can focus on one class to test at a time and mock everything else out.
Miranda testing has also been simplified by the use of BlockingQueues and Messages. A class method typically does a few things, then sends out a Message in response to an event. Thus the test has to check that it took those actions. The tests tend to be simple things because of this.