Add "node" to Jest's moduleFileExtensions (facebook#2738)

Jest's default value for this option is ["js", "json", "jsx", "node"].
CRA's current value is ["web.js", "js", "json", "web.jsx", "jsx"], which
is missing one of default ones: "node". This change fixes that.

This "node" extension is used in packages that use native code.

packages/react-scripts/scripts/utils/createJestConfig.js

@@ -43,7 +43,7 @@ module.exports = (resolve, rootDir, isEjecting) => {
     moduleNameMapper: {
       '^react-native$': 'react-native-web',
     },
-    moduleFileExtensions: ['web.js', 'js', 'json', 'web.jsx', 'jsx'],
+    moduleFileExtensions: ['web.js', 'js', 'json', 'web.jsx', 'jsx', 'node'],
   };
   if (rootDir) {
     config.rootDir = rootDir;