Improve tests for JavaScript (#3496)

- Upgrade dependencies
    - chai (3.5.0 -> 4.0.1)
    - chai-enzyme (0.6.1 -> 0.7.1)
    - sinon (2.2.0 -> 2.3.2)
- Change extensions from .jsx to .js
- Don't assign `React` to `global`
- Check code format using ESLint

spec/javascript/components/avatar.test.js

@@ -0,0 +1,20 @@
+import { expect } from 'chai';
+import { render } from 'enzyme';
+import React from 'react';
+import Avatar from '../../../app/javascript/mastodon/components/avatar';
+
+describe('<Avatar />', () => {
+  const src = '/path/to/image.jpg';
+  const size = 100;
+  const wrapper = render(<Avatar src={src} animate size={size} />);
+
+  it('renders a div element with the given src as background', () => {
+    expect(wrapper.find('div')).to.have.style('background-image', `url(${src})`);
+  });
+
+  it('renders a div element of the given size', () => {
+    ['width', 'height'].map((attr) => {
+      expect(wrapper.find('div')).to.have.style(attr, `${size}px`);
+    });
+  });
+});