I've skimmed the article and still have no clue what you guys are talking about. Is test harness another word for end to end testing?

Did you see this helpful link: https://littlegreenviper.com/testing-harness-vs-unit/. According to that usage of "test harness", it's not the same as a unit test. It's a way of driving code manually to explore different usages and to recreate certain kinds of bugs. It is used to complement unit tests.

Well, I thought I explained it well. Frankly, I'm shocked that folks don't know what test harnesses are. They are one of the oldest constructs in Software Engineering. In the days of old, we often called them "Unit Tests."

If you want some concrete examples, here you go:

Here's my "Spinner" widget:

https://github.com/RiftValleySoftware/RVS_Spinner

This is the four test harness apps:

https://github.com/RiftValleySoftware/RVS_Spinner/tree/main/...

This is my checkbox widget:

https://github.com/RiftValleySoftware/RVS_Checkbox

Here's the test harness app:

https://github.com/RiftValleySoftware/RVS_Checkbox/tree/main...

There's more, but these should be enough to illustrate the point.

"Compared to unit tests, a test harness is a [usually] simple standalone application that incorporates the system under test, and presents a user interface. Since it is for testing, it may expose internal metrics and allow the user to access parts of the code that are usually hidden."

^ from https://littlegreenviper.com/testing-harness-vs-unit/