test: restore GCC 8 compatibility in LWG-4222 tests

Author: X1aomu

tests/test_expected/lwg_4222_tests.cpp

@@ -10,7 +10,7 @@ namespace
 struct ConstructibleFromUnexpect
 {
     ConstructibleFromUnexpect() = default;
-    explicit ConstructibleFromUnexpect([[maybe_unused]] unexpect_t ut) {}
+    explicit ConstructibleFromUnexpect(unexpect_t) {}
 };
 
 } // namespace