function Runnable(title, fn) {
  this.title = title;
  this.fn = fn;
  this.async = fn && fn.length;
  this.sync = ! this.async;
  this._timeout = 2000;
  this._slow = 75;
  this.timedOut = false;
}
Initialize a new `Runnable` with the given `title` and callback `fn`.

@param {String} title
@param {Function} fn
@api private