title: Mock delay() RxJS with Jest
body: <p>Is there easy way to mock <code>delay()</code> method of RxJS in an observable with a fake time for example ? </p>
  
  <p>I have this method :</p>
  
  <pre><code>register(user) {
    return this._checkLog(user).delay(500).flatMap( ... )
  }
  </code></pre>
  
  <p>when i remove <code>delay()</code> method, my tests from _register() all success.</p>