src: remove usage of `std::shared_ptr<T>::unique()`
std::shared_ptr<T>::unique() has been removed in C++20, so this change uses std::shared_ptr<T>::use_count() instead which is available in C++20.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
std::shared_ptr<T>::unique() has been removed in C++20, so this change uses std::shared_ptr<T>::use_count() instead which is available in C++20.