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/button.test.js

@@ -0,0 +1,71 @@
+import { expect } from 'chai';
+import { shallow } from 'enzyme';
+import sinon from 'sinon';
+import React from 'react';
+import Button from '../../../app/javascript/mastodon/components/button';
+
+describe('<Button />', () => {
+  it('renders a button element', () => {
+    const wrapper = shallow(<Button />);
+    expect(wrapper).to.match('button');
+  });
+
+  it('renders the given text', () => {
+    const text = 'foo';
+    const wrapper = shallow(<Button text={text} />);
+    expect(wrapper.find('button')).to.have.text(text);
+  });
+
+  it('handles click events using the given handler', () => {
+    const handler = sinon.spy();
+    const wrapper = shallow(<Button onClick={handler} />);
+    wrapper.find('button').simulate('click');
+    expect(handler.calledOnce).to.equal(true);
+  });
+
+  it('does not handle click events if props.disabled given', () => {
+    const handler = sinon.spy();
+    const wrapper = shallow(<Button onClick={handler} disabled />);
+    wrapper.find('button').simulate('click');
+    expect(handler.called).to.equal(false);
+  });
+
+  it('renders a disabled attribute if props.disabled given', () => {
+    const wrapper = shallow(<Button disabled />);
+    expect(wrapper.find('button')).to.be.disabled();
+  });
+
+  it('renders the children', () => {
+    const children = <p>children</p>;
+    const wrapper = shallow(<Button>{children}</Button>);
+    expect(wrapper.find('button')).to.contain(children);
+  });
+
+  it('renders the props.text instead of children', () => {
+    const text = 'foo';
+    const children = <p>children</p>;
+    const wrapper = shallow(<Button text={text}>{children}</Button>);
+    expect(wrapper.find('button')).to.have.text(text);
+    expect(wrapper.find('button')).to.not.contain(children);
+  });
+
+  it('renders style="display: block; width: 100%;" if props.block given', () => {
+    const wrapper = shallow(<Button block />);
+    expect(wrapper.find('button')).to.have.className('button--block');
+  });
+
+  it('renders style="display: inline-block; width: auto;" by default', () => {
+    const wrapper = shallow(<Button />);
+    expect(wrapper.find('button')).to.not.have.className('button--block');
+  });
+
+  it('adds class "button-secondary" if props.secondary given', () => {
+    const wrapper = shallow(<Button secondary />);
+    expect(wrapper.find('button')).to.have.className('button-secondary');
+  });
+
+  it('does not add class "button-secondary" by default', () => {
+    const wrapper = shallow(<Button />);
+    expect(wrapper.find('button')).to.not.have.className('button-secondary');
+  });
+});