Fix flaky test

What's really needed is to wait after starting the cryptcat listener. Otherwise the second cryptcat process might start before the listener is ready and then nothing works.

Then we also need to wait after sending the message, so that it has time to arrive to the listeners.

We can do that with sleep, but we can do better and use '-S' (check for socket) and '-s' (check for non-empty file).

Finally, waiting for process to terminate with 'wait' is just good practice, but I don't think it's really required.