I believe unit tests existed. I had a test training in 2000 and things were pretty systematic already then. Not 100% sure whether the exact term was used then.
Edit: JUnit is from 1997. So the name was definitely in use in 2003. I attended a TDD tutorial before 2004 (don't remember the exact year). CI wasn't a thing yet, so you executed your unit tests manually. /edit
Do unit tests exist in the kernel today? There is some (or some would say a lot) of automatic testing for the kernel, but I don't remember seeing a single unit test.
