Moving App into a directory and adding tests

src/components/App/__snapshots__/test.js.snap

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+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Message rendering 1`] = `
+<Message
+  heading="React App"
+>
+  <p>
+    Placeholder app content
+  </p>
+</Message>
+`;

src/components/App/test.js

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+import React from 'react';
+import { shallow } from 'enzyme';
+
+import App from './index';
+
+test('Message rendering', () => {
+  const component = shallow(
+    <App/>
+  );
+  expect(component).toMatchSnapshot();
+});