[aastronaut]

To add to this, for me personally the difficult part in automated test isn't the definition of expected output, but the retrieval of actual output in an automated way. Test frameworks can and will only focus on a limited set of interaction (Playwright for DOM, detox for mobile etc.).

I think Brendan Gregg's books[0][1] are quite inspiring for an understanding of a system with its inner workings, and provide information about various tools one can use to dissect a system and get some actual output in an automated way.

[0]: https://www.brendangregg.com/systems-performance-2nd-edition...

[1]: https://www.brendangregg.com/bpf-performance-tools-book.html