Add CI smoke test for the package skeleton

Making modifications to NID skeletons was always tricky - we usually copied working makefiles from existing projects where the changes were already implemented. Sometimes these changes broke things because of some other parts missing, but we did not notice this because we didn't test the skeletons. This is bad! The simple test exercises the most common testenv recipes:

• build
• test, which in turn runs testenv-start, testenv-test and testenv-stop