Triggerable controller event

Stem's integ tests presently use tor's BW events to check its ability to receive asynchronous events. This is because BW events are reliably emitted each second, and as far as I'm aware it's the only way to reliably get an event.

This is essentially a sleep in our tests. Better would be to have an event we can trigger on demand. This would speed up our tests a little and improve their reliability...

https://gitweb.torproject.org/stem.git/tree/test/integ/control/controller.py#n36

Sebastian had the neat idea for new PING event type that can be triggered by calling a controller method. But perhaps we already have a triggerable event I'm overlooking?