update App.jsx and improve coverage

abstract LogoutButton to component

__tests__/components/LogoutButton.test.jsx

@@ -0,0 +1,27 @@
+import React from 'react';
+import { mount } from 'enzyme';
+import LogoutButton from '../../src/components/LogoutButton';
+
+let buttonWrapper;
+let buttonElem;
+let userIconElem;
+
+const findChildren = () => {
+  buttonElem = buttonWrapper.find('Button');
+  userIconElem = buttonWrapper.find('UserIcon');
+};
+
+describe('<LogoutButton />', () => {
+  test('initially renders without crashing', () => {
+    const onDevLogout = jest.fn();
+    buttonWrapper = mount(<LogoutButton onDevLogout={onDevLogout} />);
+    findChildren();
+    expect(buttonWrapper.length).toBe(1);
+    expect(buttonElem.length).toBe(1);
+    expect(userIconElem.length).toBe(1);
+    buttonElem.simulate('keyDown', { keyCode: 40, which: 40 });
+    expect(onDevLogout).toHaveBeenCalledTimes(0);
+    buttonElem.simulate('keyDown', { keyCode: 13, which: 13 });
+    expect(onDevLogout).toHaveBeenCalledTimes(1);
+  });
+});