function Runnable (title, fn) {
  this.title = title;
  this.fn = fn;
  this.body = (fn || '').toString();
  this.async = fn && fn.length;
  this.sync = !this.async;
  this._timeout = 2000;
  this._slow = 75;
  this._enableTimeouts = true;
  this.timedOut = false;
  this._trace = new Error('done() called multiple times');
  this._retries = -1;
  this._currentRetry = 0;
  this.pending = false;
}
Initialize a new `Runnable` with the given `title` and callback `fn`.

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