This initializes ./test/default.nix which is referred to by the
flake.nix under the `checks` attribute. This default.nix should point to
all future tests, where they can be looked up and ran like:
nix build .#checks.x86_64-linux.vmTest
The test included is a simple NixOS VM Test. It uses Nixinate to deploy
a machine with `services.nginx.enable = true` set, and tests whether
nginx.service is started and reachable after deployment.
