You can of course test the UI by just reporting the file system to be full, but when you want to test your low-level stuff you would be resorting to faking sysc

You can of course test the UI by just reporting the file system to be full, but when you want to test your low-level stuff you would be resorting to faking syscalls. That is of course doable, but why do that when you can just create the situation directly.