title: Node-config custom-environment-variables not overridding default.json during test
body: <p>Node-config's custom-environment-variables.json doesn't seem to be overriding values in default.json when NODE_ENV is set to 'test.' Here's an extremely simple config:</p>
  
  <p>config/default.json:</p>
  
  <pre><code>{
    "jwtPrivateKey": "defaultkey"
  }
  </code></pre>
  
  <p>config/custom-environment-variables.json:</p>
  
  <pre><code>{
   "jwtPrivateKey": "JWTPRIVATEKEY"
  }
  </code></pre>
  
  <p>config/test.json:</p>
  
  <pre><code>{}
  </code></pre>
  
  <p>app.js:</p>
  
  <pre><code>console.log('NODE_ENV': + process.env.NODE_ENV);
  console.log('Env: ' + process.env.JWTPRIVATEKEY);
  console.log("Config: " + config.get('jwtPrivateKey'));
  </code></pre>
  
  <p>This works as expected: I get NODE_ENV=undefined (so node-config will default to 'development'), &amp; the next two lines both print the correct value set in the environment variable.  However, if I try to run a simple test with Jest &amp; print the same:</p>
  
  <p>tests/some.test.js:</p>
  
  <pre><code>test('Some Test', () =&gt; {
      console.log('NODE_ENV:' + process.env.NODE_ENV);
      console.log('Env: ' + process.env.JWTPRIVATEKEY);
      console.log("Config: " + config.get('jwtPrivateKey'));
  });
  </code></pre>
  
  <p>I get NODE_ENV=test, Env=the correct value set in the environment variable...but Config=defaultKey.</p>
  
  <p>In other words: although it shows that the environment variable is set to the right value, node-config doesn't seem to be pulling it in through custom-environment-variables.json.</p>
  
  <p>Is this a bug?  I've scoured the documentation but been unable to find any reason for this discrepancy.</p>