vscode: set profile name in testbed (#980)

Previously, the theme was not enabled in the testbed because there were
no existing profiles.

modules/vscode/testbeds/default.nix

@@ -16,5 +16,7 @@ in
       enable = true;
       inherit package;
     };
+
+    stylix.targets.vscode.profileNames = [ "default" ];
   };
 }