tests: Move NixOS tests to tests/nixos

This will allow contributors to find them more easily.

tests/nixos/containers/systemd-nspawn.nix

@@ -0,0 +1,78 @@
+{ nixpkgs }:
+
+let
+
+  machine = { config, pkgs, ... }:
+    {
+      system.stateVersion = "22.05";
+      boot.isContainer = true;
+      systemd.services.console-getty.enable = false;
+      networking.dhcpcd.enable = false;
+
+      services.httpd = {
+        enable = true;
+        adminAddr = "nixos@example.org";
+      };
+
+      systemd.services.test = {
+        wantedBy = [ "multi-user.target" ];
+        after = [ "httpd.service" ];
+        script = ''
+          source /.env
+          echo "Hello World" > $out/msg
+          ls -lR /dev > $out/dev
+          ${pkgs.curl}/bin/curl -sS --fail http://localhost/ > $out/page.html
+        '';
+        unitConfig = {
+          FailureAction = "exit-force";
+          FailureActionExitStatus = 42;
+          SuccessAction = "exit-force";
+        };
+      };
+    };
+
+  cfg = (import (nixpkgs + "/nixos/lib/eval-config.nix") {
+    modules = [ machine ];
+    system = "x86_64-linux";
+  });
+
+  config = cfg.config;
+
+in
+
+with cfg._module.args.pkgs;
+
+runCommand "test"
+  { buildInputs = [ config.system.path ];
+    requiredSystemFeatures = [ "uid-range" ];
+    toplevel = config.system.build.toplevel;
+  }
+  ''
+    root=$(pwd)/root
+    mkdir -p $root $root/etc
+
+    export > $root/.env
+
+    # Make /run a tmpfs to shut up a systemd warning.
+    mkdir /run
+    mount -t tmpfs none /run
+    chmod 0700 /run
+
+    mount -t cgroup2 none /sys/fs/cgroup
+
+    mkdir -p $out
+
+    touch /etc/os-release
+    echo a5ea3f98dedc0278b6f3cc8c37eeaeac > /etc/machine-id
+
+    SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1 \
+      ${config.systemd.package}/bin/systemd-nspawn \
+      --keep-unit \
+      -M ${config.networking.hostName} -D "$root" \
+      --register=no \
+      --resolv-conf=off \
+      --bind-ro=/nix/store \
+      --bind=$out \
+      --private-network \
+      $toplevel/init
+  ''