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.