add sinon; add basic Button component test

spec/javascript/components/button.test.jsx

@@ -0,0 +1,14 @@
+import { expect } from 'chai';
+import { shallow } from 'enzyme';
+import sinon from 'sinon';
+
+import Button from '../../../app/assets/javascripts/components/components/button'
+
+describe('<Button />', function() {
+  it('simulates click events', function() {
+    const onClick = sinon.spy();
+    const wrapper = shallow(<Button onClick={onClick} />);
+    wrapper.find('button').simulate('click');
+    expect(onClick.calledOnce).to.equal(true);
+  });
+});