Method cleanupAllInstances

tests/test_smart_ptr.cpp:140–146

    int value;

    static void cleanupAllInstances() {
        auto tmp = std::move(pointer_set<MyObject4>());
        pointer_set<MyObject4>().clear();
        for (auto *o : tmp) {
            delete o;
        }
    }

private:
    ~MyObject4() {