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/setup.js

@@ -15,8 +15,5 @@ Object.keys(document.defaultView).forEach((property) => {
 });
 
 global.navigator = {
-  userAgent: 'node.js'
+  userAgent: 'node.js',
 };
-
-var React    = window.React    = global.React    = require('react');
-var ReactDOM = window.ReactDOM = global.ReactDOM = require('react-dom');