title: Mock in Jest: Warning: Each child in a list should have a unique "key" prop
body: <p>I have written tests that trigger the <code>React</code> warning</p>
  
  <blockquote>
    <p><code>Warning: Each child in a list should have a unique "key" prop.</code></p>
  </blockquote>
  
  <p>I know why this warning is there, but I need to test for this case. But I don't want to see the warning when I run my tests.</p>
  
  <p>How can I write a mock that hides that message or even let's me spy on it so I can see it's there?</p>
  
  <p>I am using <code>React</code> and <code>Jest</code></p>