Create E2E testing utilities for addons

[eirikurn]

It should be simple to create a folder with .neutrinorc.js and app.js files which are built and run in node and the browser.

It should be possible to assert the state of the dom before and after hydration as well as assert things inside rendering. E.g. a set of flags that should be set during a test run. If any part fails, some of the flags wouldn't be set so the test fails with a "friendly message".

It should be possible to test dev and prod setup.

Here are some assertions for different addons.

Tux
- assert babel es6 and jsx
- assert extract css
- assert error overlay?
- assert server render
- assert client render

React router
- assert server render route for url
- assert client render route for url

Styled components
- assert server render of styles
- assert client render of styles
- assert reuse of styles, same ids

Async bootloader
- assert async callback on server and client

Async jobs
- assert ssr of async job
- assert hydration in browser

Helmet
- assert ssr of meta tags