@Override public String toString() {
    return "[Bus \"" + identifier + "\"]";
  }
Test constructor which allows replacing the default {@code HandlerFinder}.

@param enforcer Thread enforcer for register, unregister, and post actions.
@param identifier A brief name for this bus, for debugging purposes.  Should be a valid Java identifier.
@param handlerFinder Used to discover event handlers and producers when registering/unregistering an object.
otto/src/main/java/com/squareup/otto/Bus.java