Expand the Testing Framework page

[bugfolder]

In backdrop/backdrop-issues#5415, the PR adds a link to the Testing Framework page on docs.b.org, which (up until a few minutes ago) looked like this:

So I've expanded it a lot (as a revision, so is revertible if needed). Here's what it looks like now.

https://docs.backdropcms.org/documentation/testing-framework

Further changes/additions/corrections are, of course, welcome, and can be suggested in this issue. Feel free to make small changes or typo corrections directly (if you have editing permissions) or suggest them here (if not).

[klonos]

Thank you so much for getting that started @bugfolder 🙏🏼