This run_test helper sets up a convention that allows
us to give really short tracebacks for errors, and
eventually we can have more control over running
individual tests. (The latter goal has some
complications, since we often intentionally leak
setup in tests.)
This takes advantage of the new function narrow_state.stream_id().
We now assume the incoming stream_id is a valid stream_id, so we
no longer need to test some of the error checking. (It's possible that
the incoming stream_id may no longer be for a stream you subscribe
to, but the nice benefit of working more in "id space" is that if
it doesn't match the narrow's stream id, we know false is a safe
return value.)