Unmount the App in the default test (#3511)

Author: gaearon

packages/react-scripts/template/src/App.test.js

@@ -5,4 +5,5 @@ import App from './App';
 it('renders without crashing', () => {
   const div = document.createElement('div');
   ReactDOM.render(<App />, div);
+  ReactDOM.unmountComponentAtNode(div);
 });