JUnit Core: All RunListeners should expose the stack trace in case of test failures

Currently if a remote unit test fails on the client side you only see the assertion message but not the stack trace (which could give more hints, why exactly the test case failed).

By default the SlingRemoteTestRunner leverages the response of the JsonRenderer to give out an error. The JsonRenderer currently does not print the stack trace. It should rather do that via Failure#getTrace() instead of just relying on Failure#toString().
That should be done for all Renderers (to also ease debugging if the JUnit servlet is directly called with a browser). The response trace should then correctly be evaluated in SlingRemoteTest.run.