Revert "nixos/lib/testing: avoid generating darwin VM tests"

This reverts commit 72155225aa.

nixos/lib/testing/default.nix

@@ -9,15 +9,13 @@ let
     };
   runTest =
     module:
-    # Infra issue: virtualization on darwin doesn't seem to work yet.
-    lib.addMetaAttrs { hydraPlatforms = lib.platforms.linux; }
-      (evalTest (
-        { config, ... }:
-        {
-          imports = [ module ];
-          result = config.test;
-        }
-      )).config.result;
+    (evalTest (
+      { config, ... }:
+      {
+        imports = [ module ];
+        result = config.test;
+      }
+    )).config.result;
 
   testModules = [
     ./call-test.nix

nixos/lib/testing/meta.nix

@@ -36,9 +36,7 @@ in
           };
           platforms = lib.mkOption {
             type = types.listOf types.raw;
-            # darwin could be added, but it would add VM tests that don't work on Hydra.nixos.org (so far)
-            # see https://github.com/NixOS/nixpkgs/pull/303597#issuecomment-2128782362
-            default = lib.platforms.linux;
+            default = lib.platforms.linux ++ lib.platforms.darwin;
             description = ''
               Sets the [`meta.platforms`](https://nixos.org/manual/nixpkgs/stable/#var-meta-platforms) attribute on the [{option}`test`](#test-opt-test) derivation.
             '';