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/display_name.test.js

@@ -0,0 +1,27 @@
+import { expect } from 'chai';
+import { render } from 'enzyme';
+import Immutable  from 'immutable';
+import React from 'react';
+import DisplayName from '../../../app/javascript/mastodon/components/display_name';
+
+describe('<DisplayName />', () => {
+  it('renders display name + account name', () => {
+    const account = Immutable.fromJS({
+      username: 'bar',
+      acct: 'bar@baz',
+      display_name: 'Foo',
+    });
+    const wrapper = render(<DisplayName account={account} />);
+    expect(wrapper).to.have.text('Foo @bar@baz');
+  });
+
+  it('renders the username + account name if display name is empty', () => {
+    const account = Immutable.fromJS({
+      username: 'bar',
+      acct: 'bar@baz',
+      display_name: '',
+    });
+    const wrapper = render(<DisplayName account={account} />);
+    expect(wrapper).to.have.text('bar @bar@baz');
+  });
+});