Write react unit tests for the app

[sdankel]

First, fix the issue with webpack + jest to get this test passing.

 FAIL  src/App.test.tsx
  ● Test suite failed to run

    Cannot find module 'file-loader?esModule=false!./src-noconflict/ext-beautify.js' from 'node_modules/ace-builds/webpack-resolver.js'

    Require stack:
      node_modules/ace-builds/webpack-resolver.js
      src/features/editor/components/Editor.tsx
      src/App.tsx
      src/App.test.tsx

      at Resolver.resolveModule (node_modules/jest-resolve/build/resolver.js:324:11)
      at Object.<anonymous> (node_modules/ace-builds/webpack-resolver.js:2:45)

Then write unit tests for features in the App.