title: node-config Config is `undefined` in typescript jest context?
body: <p>I have a typescript module that works in dev and production. it uses <a href="https://github.com/lorenwest/node-config" rel="nofollow noreferrer">https://github.com/lorenwest/node-config</a> I am trying to import it into jest to write tests against and it throws an error that indicates the config object is undefined</p>
  
  <pre><code>TypeError: Cannot read property 'get' of undefined
  at Object.&lt;anonymous&gt; (src/email/email.service.ts:1877:43)
  </code></pre>
  
  <p>from</p>
  
  <pre><code>import config from 'config'
  console.log('**********CONFIG GET*********', config); // undefined
  const sendgridToken: string = config.get('socialApi.value') // throws error
  </code></pre>
  
  <p>I'd expect config to be defined its the default export of the config module</p>
  
  <p>environment:</p>
  
  <pre><code>node-config version: config@3.2.4
  node-version: 12.13.1
  tsc -v
  Version 3.7.4
  ts-jest@24.3.0
  </code></pre>