Adding tests for App component

jest/preprocess.js

@@ -1,5 +1,6 @@
 const babelOptions = {
-  presets: ['babel-preset-gatsby']
+  presets: ['babel-preset-gatsby'],
+  plugins: ['dynamic-import-node']
 };
 
 module.exports = require('babel-jest').createTransformer(babelOptions);