Add automated test

[srid]

Resolves #25. To be squash merged.

• Add ./test that aims to use all features of haskell-flake
• Add ./test/test.sh that verifies that the devshell of ./test is as one would expect
• Add ./runtest.sh at project root to run the full test
• Run the same ^ in Github Actions

I'm not entirely happy with this approach; in particular, test.sh seems pretty hacky. But it works. Not sure if it will scale.

I just would like to have some sort of automated tests before doing substantial changes to haskell-flake.

[roberth]

I'm not entirely happy with this approach

Me neither, but it seems to get the job done.
Improvements are kind of blocked Flakes improvements like

• Development only flake inputs or different category of inputs NixOS/nix#6124
• allow private flake inputs NixOS/nix#7028
• Reuse input lock files NixOS/nix#7730
• possibly Lazy trees NixOS/nix#6530 for .. input urls?

[srid]

Yea, let's go with it for now and make it perfect when the time is right.