title: What is the best way to assert for a scala method whose return type is Future[Unit]
body: <p>I have a method. This method may return Future.failed(.....) or Future.successful(()).</p>
  
  <p>def calculate(x: Int, y: Int): Future[Unit] = {
  ........
  }</p>
  
  <p>Now I need to test this method. What is the best way to assert for the test which verifies  <code>Future.successful(())</code> case .</p>