title: Prevent "test/expect/etc is not defined" errors when using Jest
body: <p>Facebook's Jest testing framework is easy to <a href="http://facebook.github.io/jest/docs/en/getting-started.html" rel="noreferrer">get started with, but the documentation</a> overlooks an <a href="https://github.com/facebook/jest/issues/3762#issuecomment-307597144" rel="noreferrer">annoying aspect</a>: test statements will be highlighted as errors by any editor that tries to warn of undefined symbols, because <code>test</code>, <code>expect</code>, and all matcher methods are not defined.</p>
  
  <p><a href="https://i.stack.imgur.com/Bw6Cf.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Bw6Cf.png" alt="WebStorm highlights errors in Jest test file"></a></p>
  
  <p>Similary, attempting to run a test file with <code>node</code> directly will fail with <code>ReferenceError: test is not defined</code>.</p>
  
  <p>What <code>require</code>/<code>import</code> statement(s) need to be added for those errors to go away?</p>