tests: add a README.md for the test framework

tests/README.md

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+# Testing the comprehensive rust course book
+
+The course material contains JS code that can break and needs to be checked for
+functionality. Examples are `theme/speaker-notes.js` or `theme/book.js`.
+
+Comprehensive Rust 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.
+
+The [Static Server Service](https://webdriver.io/docs/static-server-service/) is
+used mainly in the [CI](../github/workflows/build.yml) to serve the book on port
+localhost:8080 such that the test runner can access it. This mode is used when
+`npm start` or `npm test` is executed.
+
+For local testing and quick iterations it is possible to use `mdbook serve`
+which creates a small HTTP server on port 3000 by default. There is a special
+config that is invoked with `npm run test-mdbook` that uses
+`http://localhost:3000`