Use NODE_ENV from environment instead of overriding it

webpack/test.js

@@ -1,4 +1,5 @@
 // Note: You must restart bin/webpack-dev-server for changes to take effect
+console.log('Running in test mode'); // eslint-disable-line no-console
 
 const merge = require('webpack-merge');
 const sharedConfig = require('./shared.js');