title: how to test redux-saga all effect using jest
body: <pre><code>function* mySaga() {
    const [customers, products] = yield all([
      call(fetchCustomers),
      call(fetchProducts)
    ])
  }
  </code></pre>
  
  <p>I want to test all effect in jest but I get: <code>Invalid attempt to destructure non-iterable instance</code></p>
  
  <p>my code is:</p>
  
  <pre><code>    const generator = mySaga()
      expect(generator.next().value).toEqual(all([
        call(fetchCustomers),
        call(fetchProducts)
      ]))
  </code></pre>