I dont think it is about discipline. Discipline is required if you're duplicating tedious work, not for creativity.

At its core, a good test will take an example and do something with it to demonstrate an outcome.

That's exactly what how to docs do - often with the exact same examples.

Logically, they should be the same thing.

You just need a (non turing complete) language that is dual use - it generates docs and runs tests.

For example:

https://github.com/crdoconnor/strictyaml/blob/master/hitch/s...

And:

https://hitchdev.com/strictyaml/using/alpha/scalar/email-and...