Issue description

While the system we have right now is good for contributing, I think each new nix expression should have a test in nixos/tests written along with it in each new PR.
We should do this retroactively where possible for all currently existing packages too.
This way, we can ensure everything works with hydra, and be alerted quickly for broken, but successfully built expressions that may not be actively maintained and (user) tested anymore.

Would it be possible to change the policy in Contributing to do this?
The flow I see is this:

1. Make new expression for program X
2. Test locally to ensure it works
3. Make test in nixos/tests for it so hydra can do this automatically
4. Submit PR after everything fits Contributing.

Is this a good idea, something we should implement? I see more benefits that drawbacks.