Okay, I can run the tests now using https://github.com/knurling-rs/app-template 🚀
Running the tests via the just CLI works!

> It is compatible with rust-analyzer's ▶ Run Test button, which means you can flash and run your tests straight from VS Code
from [here](https://github.com/knurling-rs/defmt/tree/main/firmware/defmt-test#defmt-test) is leading to the error message above in my case. Unfortunately, I was too focused on just clicking the overlay button..