clanServices: Move admin test to appropriate location

Move the admin clanService test to the admin clan service.
The tests should live close to the definition of the service, so that
debugging and discoverability is better.

clanServices/admin/flake-module.nix

@@ -1,6 +1,17 @@
-{ lib, ... }:
+{ lib, self, ... }:
 {
   clan.modules = {
     admin = lib.modules.importApply ./default.nix { };
   };
+  perSystem =
+    { pkgs, ... }:
+    {
+      checks = lib.optionalAttrs (pkgs.stdenv.isLinux) {
+        admin = import ./tests/vm/default.nix {
+          inherit pkgs;
+          clan-core = self;
+          nixosLib = import (self.inputs.nixpkgs + "/nixos/lib") { };
+        };
+      };
+    };
 }