Most applications I've ever worked on talk to the internet or to other servers on the network, you can't close your eyes and pretend everything runs on one mach

Most applications I've ever worked on talk to the internet or to other servers on the network, you can't close your eyes and pretend everything runs on one machine. You eventually have to test all of your software. Certainly, you can optimize some of your tests by caching them! But they should be fast anyway, because your software should be fast. The software I currently work on has massive test suites containing around 50k tests that run in about 3 minutes per suite on my PC. It's time consuming, I suppose, but caching would mean that I'm not actually flushing out latent bugs. Instead, we run all of the tests every time and do things like randomize the execution order, so we find bugs before customers do. At the point where your software interacts with threads (or the kernel!) you've already brought in non-determinism and trying to act like your tests are really deterministic is naive.