I do that too. Although I guess once I’m testing code in an actual game I think of that as integration testing instead of unit testing, but that’s (mostly) just semantics.
Testing low-level stuff in isolation isn’t intended to be a replacement for testing how everything works together. It’s because when you put everything together for the first time it won’t work, barring miracles. And when your first build isn’t an event of major theological significance, the fact that you can verify that each of the bits works by itself and most of them work between themselves, then that tells you that the locus of the problem is in your stuff.