add preact snapshot test

[paranoidjk]

preactjs/preact#560
preactjs/preact-render-to-string#16
enzymejs/enzyme#742

[benjycui]

@yesmeck 其实单测可不可以这样写：

import React from '../shared/ReactOrPreact';

...

// ReactOrPreact

export default (function () {
  if (ENV === 'xxx') {
    return Preact
  }
  return React
})();

这样无论 Snapshot 还是单测都可以同时测试 Preact 和 React

[yesmeck]

应该可以把 React mock 掉，http://facebook.github.io/jest/docs/en/manual-mocks.html#content

在 Mock 里判断用哪个版本的 React

[yesmeck]

但是需要 React 和 Preact 生成的 snapshot 一致。

[paranoidjk]

i will try this first https://github.com/ruyadorno/preact-jest-snapshot-test-boilerplate

[paranoidjk]

snapshot 跑通了，#1306

还剩一个问题是 jest-serializer-html-string 没有把 html 元素的属性换行排列，所以与 react snaoshot 不一致，这个抽空过去 PR fix 掉。

[paranoidjk]

preact, inferno, react-lite 几个库跑 react 官方用例：

Lucifier129/react-core-unit-testing#1

https://medium.com/@guyingjie129/why-you-shouldnt-use-preact-and-inferno-to-replace-react-4000ffb44572

[paranoidjk]

preactjs/preact-render-to-string#16 (comment)