message: add mount() testing utility
diff: import renderer from 'react-test-renderer'
  +import enzyme from 'enzyme'
  +import Adapter from 'enzyme-adapter-react-16'
  /**
  * Render the component (a React.createElement() or JSX expression)
  @@ -30,3 +32,11 @@ export function renderClasses(component) {
  } = render(component)
  return className ? className.trim().split(' ') : []
  }
  +
  +let enzymeConfigured = false
  +export function mount(component) {
  + if (!enzymeConfigured) {
  + enzyme.configure({adapter: new Adapter()})
  + }
  + return enzyme.mount(component)
  +}
  