Implement test cases for Amazon AMIs

[hbjydev]

I want to start putting in some 'unit tests' for our VMs made with this repo to ensure they'll run correctly, not just build. I've been making some config typos quite often in Alloy, for instance, which then means I have to go back and do another AMI update cycle to build an updated image, then wait for our TF to reconcile the new image. That's a long feedback loop when there could just have been some tests in the first place.

NixOS does this pretty nicely in nixpkgs already, so I'd like to do similar stuff for each of our mixins in this repo.

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See https://nixos.wiki/wiki/NixOS_Testing_library for docs