Add a test framework to test the JS part of the course

[michael-kerscher]

This is a first draft for a test framework for testing the JS part of the book that is discussed in #2462

It is using webdriverIO and the webdriverIO Expect API in combination with Mocha. WebdriverIO is taking care of accessing the webpage with a real browser and can access the state of the page so behavior can be asserted.

Currently only a small test for the speaker-notes implementation demos the functionality.

The Static Server Service is used to serve the book in a way that the test runner can access it.

A CI integration can look like https://webdriver.io/docs/githubactions/ and is implemented with a headless setup. In CI it uses the language variable to set environment variable that configures where the built book should be mounted from

Fixes #2462.

[mgeisler]

This looks really cool from a quick local test run!

[mgeisler]

Small things to add here or in a later PR:

• a README which briefly explains the setup and in particular has links to documentation for the
  • the Mocha unit test framework and
  • the WebdriverIO test runner (just so I don't have to look it up myself when I want to add more tests)
• when we have a README in the directory, we should then also add a link to to other relevant documentation so people know to update the tests
• perhaps a default command for npm so that npm run does the right thing by default? If I remember correctly from my web developer days, that's possible 😄
• would it be possible to have a setup for local development where the tests run against the server on http://localhost:3000 started by mdbook serve? Using the HTML on disk is great for GitHub CI, but I think it would be cool to iterate with mdbook serve locally

Feel free to do all this here or in later PRs, the existing setup is already a great improvement on what we have right now.

[michael-kerscher]

Thinking about mdbook serve, yes there is a benefit to this I have not thought of by not having to recreate the book.
This looks a bit more difficult to support two different setups though. Either I'm using mdbook serve in CI as well or I modify the config so it can be configured by the CI by setting a environment variable

[michael-kerscher]

I included a second config, deriving from the regular config that uses http://localhost:3000 (mdbook serve). The CI still uses the static file service to test exactly what is used in production.
A Readme is now also available.
I'm open to discuss if npm start should be developer friendly and use the npm test-mdbook or npm test. Currently it is testing with the static service

[mgeisler]

Looks awesome, please merge this so we can get it into production!