I love authoring development test notebooks in Mathematica. It offers the most powerful testing environment I’ve ever used!
You can get really quick feedback during development by running every main function or method in the .wl Paclet code files in a Wolfram Notebook, embedded with easy to read commentary and narration. Nice clean presentation format, and you can easily embed plots, tables, anything you want. All generated directly from your Packet code.
I also love the way you can plonk just about any kind of image you like in it and resize them nicely. Super range of supported formats for heaps for media types.
The interactive Chapter/Section system sometimes need a bit of baby-sitting, but once you are used to it, you can make really clean well grouped content.
You get a huge amount of reuse from them too. You can easily turn them into slides (stills), the also use slides in screencast videos. Or screencast video capture (I just love and adore ScreenFlow) in small runs. Great development record!