Add test case for uncertain promotion scenario

If our active source disconnects at the same time as we're trying to
promote a source that's third on our list, our request can be overridden by the
promotion logic in the source disconnect handler.  Instead of "source3" ending 
up active, we get "source2".

I'm not sure that's a bug worth caring about, but I figured checking in a test
case to start discussion couldn't hurt.