add jsdom; add basic Avatar component test

spec/javascript/components/avatar.test.jsx

@@ -0,0 +1,12 @@
+import { expect } from 'chai';
+import { render } from 'enzyme';
+
+import Avatar from '../../../app/assets/javascripts/components/components/avatar'
+
+describe('<Avatar />', function() {
+  it('renders an img with the given src', function() {
+    const src = '/path/to/image.jpg';
+    const wrapper = render(<Avatar src={src} size={100} />);
+    expect(wrapper.find(`img[src="${src}"]`)).to.have.length(1);
+  });
+});