function start () {
    self.started = true;
    self.emit('start');
    self.runSuite(rootSuite, function () {
      debug('finished running');
      self.emit('end');
    });
  }
Run the root suite and invoke `fn(failures)`
on completion.

@param {Function} fn
@return {Runner} for chaining
@api public
@param {Function} fn
@return {Runner} Runner instance.