quicklisp-to-nix: check that the package is loadable

pkgs/development/lisp-modules/quicklisp-to-nix/nix-package.emb

···
     url = ''<% @var url %>'';
     sha256 = ''<% @var sha256 %>'';
   };
+
+  overrides = x: {
+    postInstall = ''
+        echo "$CL_SOURCE_REGISTRY"
+        NIX_LISP_PRELAUNCH_HOOK='nix_lisp_run_single_form "(asdf:load-system :<% @ifnotempty testname %><% @var testname %><% @else %><% @var name %><% @endif %>)"' "$out/bin/<% @var filename %>-lisp-launcher.sh" ""
+    '';
+  };
 }