NixOS Integration tests: Re-enable for macOS

nixos/lib/testing/meta.nix

···
           };
           platforms = lib.mkOption {
             type = types.listOf types.raw;
-            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.
             '';

pkgs/build-support/trivial-builders/test/default.nix

···
   concat = callPackage ./concat-test.nix {};
   linkFarm = callPackage ./link-farm.nix {};
   overriding = callPackage ../test-overriding.nix {};
-  # VM test not supported beyond linux yet
-  references =
-    if stdenv.hostPlatform.isLinux
-    then references
-    else {};
+  inherit references;
   writeCBin = callPackage ./writeCBin.nix {};
   writeClosure-union = callPackage ./writeClosure-union.nix {
     inherit (references) samples;