Document assertions & testing library

[oleiade]

What?

This Pull Request adds documentation for the k6-testing jslib, and adds an Assertion category to the using-k6 category.

The initial plan was to introduce the test framework as a k6/testing module. The idea is not abandoned, but we'd rather wait to see how our k6 ecosystem improvement story unfolds before commiting. As such, because assertions where designed to be a prime feature of k6, I took the liberty to add it next to checks and thresholds in the documentation, but adding an "admonition" that it is a preview feature.

This is a massive PR, apologies for this, there are no easy way to document a massive feature without spawning a lot of content at once. I suggest reviewing this by starting the docs locally, and going through it 🙇🏻

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.
• I have made my changes in the docs/sources/k6/next folder of the documentation.
• I have made my changes in the docs/sources/k6/next folder of the documentation.

References

grafana/k6#4805