Suppress unused variable warning

Author: XuehaiPan

tests/test_multiple_interpreters/mod_per_interpreter_gil_with_singleton.cpp

@@ -51,13 +51,15 @@ class MySingleton {
     static void init() {
         // Ensure the singleton is created
         auto &instance = get_instance();
+        (void) instance; // suppress unused variable warning
         assert(instance.objects.size() == 7);
         // Register cleanup at interpreter exit
         py::module_::import("atexit").attr("register")(py::cpp_function(&MySingleton::clear));
     }
 
     static void clear() {
         auto &instance = get_instance();
+        (void) instance; // suppress unused variable warning
         assert(instance.objects.size() == 7);
         for (const auto &obj : instance.objects) {
             obj.dec_ref();