To make sure that I did not break anything, I ran the test suite for the BankAccount sample application. The tests for classes that previously extended HashMap (Response, for instance) still passed. Of course, the tests for HashMap were now broken because trait cannot be instantiated. Suddenly I had to think about making traits testable.
He's worked up a way to test them by creating a test just for the trait and using the new "getObjectForTrait" method to instantiate an object. It returns a mocked up version of the object that can be fully interacted with. You'll need to get PHPUnit 3.6 to be able to use this feature, though.