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/loading_indicator.test.js

@@ -0,0 +1,8 @@
+import { expect } from 'chai';
+import { shallow } from 'enzyme';
+import React from 'react';
+import LoadingIndicator from '../../../app/javascript/mastodon/components/loading_indicator';
+
+describe('<LoadingIndicator />', () => {
+
+});