Add a test framework to test the JS part of the course (#2471)

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](https://webdriver.io/) and the webdriverIO
[Expect API](https://webdriver.io/docs/api/expect-webdriverio/) in
combination with [Mocha](https://mochajs.org/). 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](https://webdriver.io/docs/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

---------

Co-authored-by: Martin Geisler <martin@geisler.net>

.gitignore

@@ -19,6 +19,7 @@ count.dat
 # Nodejs Files (dprint)
 node_modules/
 package.json
+!tests/package.json
 pnpm-lock.yaml
 
 # Crowdin Config (Contains API Keys)