We also remove app.spec.ts because it will import the
serviceworker-webpack-plugin plugin and thus cannot work in jest any
more because of this error:

    serviceworker-webpack-plugin: It seems that your are importing
      "serviceworker-webpack-plugin/lib/runtime" without using the plugin.
      Makes sure that your webpack configuration is correct.

      at Object.<anonymous> (node_modules/serviceworker-webpack-plugin/lib/runtime.js:5:7)
      at Object.<anonymous> (src/app.ts:2185:68)
      at Object.<anonymous> (test/unit/app.spec.ts:5:13)