tests: add editable test

[henryiii]

@jwittbrodt or @maichmueller, any idea how to make this new test fail?

[henryiii]

I would hope this new test fails, and then https://github.com/scikit-build/scikit-build/pull/582 would be required to fix it.

[maichmueller]

@henryiii I could not get scikit-build to fail the editable install with the minimal python only package example from #546 . However, it did fail for a minimal cpp extension (pybind11) with subpackages. I have added that test case to my PR and I am testing whether the current skbuild version fails on that in the workflows. If it does, then I will re-add my fix and see if that resolves it.

[jwittbrodt]

Thanks a lot for looking into this. I also can't get my pure python example to fail anymore, seems some new version somewhere fixed that issue. But as @maichmueller said, it still fails for me if a pybind11 extension is involved.