done(failures, callback) {
    callback(SerializableWorkerResult.create(this.events, failures));
    this.events = []; // defensive
  }
Calls the {@link Mocha#run} callback (`callback`) with the test failure
count and the array of {@link BufferedEvent} objects. Resets the array.

This is called directly by `Runner#run` and should not be called by any other consumer.

Subclasses could override this.

@param {number} failures - Number of failed tests
@param {Function} callback - The callback passed to {@link Mocha#run}.
@public