message: add loadCSS() and unloadCSS() testing helpers
diff: import React from 'react'
  +import {promisify} from 'util'
  import renderer from 'react-test-renderer'
  import enzyme from 'enzyme'
  import Adapter from 'enzyme-adapter-react-16'
  import {ThemeProvider} from 'emotion-theming'
  import {default as defaultTheme} from '../theme'
  +const readFile = promisify(require('fs').readFile)
  +
  enzyme.configure({adapter: new Adapter()})
  export function mount(component) {
  @@ -122,3 +125,23 @@ export function getClasses(node) {
  const className = getClassName(node)
  return className ? className.trim().split(/ +/) : []
  }
  +
  +export function loadCSS(path) {
  + return readFile(require.resolve(path), 'utf8')
  + .then(css => {
  + const style = document.createElement('style')
  + style.setAttribute('data-path', path)
  + style.textContent = css
  + document.head.appendChild(style)
  + return style
  + })
  +}
  +
  +export function unloadCSS(path) {
  + const style = document.querySelector(`style[data-path="${path}"]`)
  + if (style) {
  + style.remove()
  + return true
  + }
  +}
  +
  