Single source of truth for luarocks test dependencies

[mrcjkb]

To run luarocks test locally (without nix), package maintainers have to add a rockspec that lists test_dependencies to the repository root (if the project has dependencies).

(See my blog post)

With the github action, this is means we have two sources of truth.
We should come up with a way to have only one source of truth (e.g. a script that generates a rockspec based on the github actions workflow yaml).

[mrcjkb]

This should be solvable by using the same core generation logic, and generating a <package>-scm-1.rockspec with test dependencies added if they have not been set.

[mrcjkb]

It might still be useful for people to have an scm rockspec in the project root, e.g. for local and nix test runs.