The "test anything protocol" was a text based system for writing tests, I think perl might still use it

I remember using it to implement the test suite for a Shakespeare language interpreter. Fun times.

https://en.wikipedia.org/wiki/Test_Anything_Protocol

Interesting approach, I can see the benefits of such approach where the md is acting like a Jupyter notebook of some sort. You place the content with documentation and it becomes a self standing unit of test.

I wonder if this can be replicated to other notions where the docs/data needed for test add more value to live with the test

I think you will like https://voiden.md/ - some what similar to what you did here.

PS: I am currently working on Voiden.