make a really fancy test harness for nix module evaluation

I've never done this before and am really proud of the code; I hope the
comments help but feel free to ask questions.

As you can see by looking at the diffs to `options.nix`, it did catch
several issues that had gotten through up to this point. I'm pretty
pleased with that. As before, `nix flake check` is all you need to do to
run it.

Change-Id: I99a550e92d7b4770e52b6aba763cff2bdc4c9287

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