title: set env variable when Running jest
body: <p>I have this variable defined on my <code>main.ts</code></p>
  
  <pre><code>const mockMode = process.env.MOCK_MODE;
  </code></pre>
  
  <p>I just create a test and set this variable to true, but on the main does not get <code>'true'</code>, but <code>'false'</code></p>
  
  <pre><code>describe('onBook', () =&gt; {
  
      // Arrange
       const mockMode = "true";
       ...
  </code></pre>