There was previously a race condition where reload.is_in_progress was
set after `activity.js` sent the presence request to the server, but
before we process the response; in that race condition, we still
shouldn't send blueslip errors to the server.
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.)
