introduce flake parts module for clan nixos tests

lib/flake-parts/clan-nixos-test.nix

@@ -0,0 +1,90 @@
+{
+  lib,
+  flake-parts-lib,
+  self,
+  inputs,
+  ...
+}:
+let
+  inherit (lib)
+    mkOption
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkPerSystemOption
+    ;
+  nixosLib = import (inputs.nixpkgs + "/nixos/lib") { };
+in
+{
+  options = {
+    perSystem = mkPerSystemOption (
+      { config, pkgs, ... }:
+      let
+        cfg = config.clan.nixosTests;
+      in
+      {
+        options.clan.nixosTests = mkOption {
+          description = "Clan NixOS tests configuration";
+          type = types.attrsOf types.unspecified;
+          default = { };
+        };
+
+        config.checks = lib.optionalAttrs (pkgs.stdenv.isLinux) (
+          let
+            # Build all individual vars-check derivations
+            varsChecks = lib.mapAttrs' (
+              name: testModule:
+              lib.nameValuePair "vars-check-${name}" (
+                let
+                  test = nixosLib.runTest (
+                    { ... }:
+                    {
+                      imports = [
+                        self.modules.nixosVmTest.clanTest
+                        testModule
+                      ];
+                      
+                      hostPkgs = pkgs;
+                    }
+                  );
+                in
+                test.config.result.vars-check
+              )
+            ) cfg;
+          in
+          lib.mkMerge [
+            # Add the VM tests as checks
+            (lib.mapAttrs (
+              _name: testModule:
+              nixosLib.runTest (
+                { ... }:
+                {
+                  imports = [
+                    self.modules.nixosVmTest.clanTest
+                    testModule
+                  ];
+                  
+                  hostPkgs = pkgs;
+                }
+              )
+            ) cfg)
+
+            # Add a single vars-check that depends on all others XXX if we ever
+            # optimize buildbot to perform better with many builds we can
+            # remove this and just run the individual vars-checks to speed up
+            # parallel evaluation.
+            (lib.optionalAttrs (varsChecks != {}) {
+              vars-check = pkgs.runCommand "vars-check-all" {
+                buildInputs = lib.attrValues varsChecks;
+              } ''
+                echo "All vars checks passed:"
+                ${lib.concatMapStringsSep "\n" (name: "echo '  ✓ ${name}'") (lib.attrNames varsChecks)}
+                touch $out
+              '';
+            })
+          ]
+        );
+      }
+    );
+  };
+}