BitD (Back in the Day), “unit tests” were independent tests that we wrote, that tested the system. It applied to pretty much any tests, including what we now call “test harnesses.” There weren’t really any “rules,” defining what a “unit test” was.

The introduction of TDD (before it, actually, as testing frameworks probably had a lot of influence), formalized what a “unit test” is.

In general, I prefer using test harnesses, over suites of unit tests[0], but I still use both.

[0] https://littlegreenviper.com/miscellany/testing-harness-vs-u...