Upstream language tests from Tvix

[Ericson2314]

I was told that Tvix has many more language tests in the same format that we use for tests/function/lang.sh. We should get those tests upstreamed as a very easy way to improve our test coverage.

[fricklerhandwerk]

The other thing we had discussed in the past is stupid and simple (but computationally expensive): Evaluate the past n Nixpkgs releases and check that all store paths remain the same. There are probably a bunch or asterisks to consider for this to actually work though.

[thufschmitt]

The other thing we had discussed in the past is stupid and simple (but computationally expensive): Evaluate the past n Nixpkgs releases and check that all store paths remain the same. There are probably a bunch or asterisks to consider for this to actually work though.

I don't think there are that many asterisks here. There's already a (rough but working) test doing pretty much that for one frozen release, we could generalize it for more releases.

The limit of that is that it won't test that much (I mean: it would already be awesome, but not enough) because the packages part of Nixpkgs doesn't exercise that many weird Nix language thingies (I don't think that test caught that many things).

#10295 might be a good complement, provided we can get it to work.