[ohgodplsno]

Nope, our Android app has caching, both for building APKs/AABs, but also for test suites. PRs run the entire test suite (on too many variants, even, I forgot to remove some unneeded stuff), what hasn't changed or hasn't been impacted by said changes simply gets a FROM-CACHE and isn't reran.

[ninkendo]

That's great for your android app, and you're lucky you have such a thing. But your experience is not generally applicable to all of software development. There are many, many, many build systems out there in the wild that do not support the ability to know what unit tests are affected by what code changes. I'd wager something like 99% of them.