Function getSingletons

tests/catch.hpp:10885–10890

    namespace {
        static auto getSingletons() -> std::vector<ISingleton*>*& {
            static std::vector<ISingleton*>* g_singletons = nullptr;
            if( !g_singletons )
                g_singletons = new std::vector<ISingleton*>();
            return g_singletons;
        }
    }

    ISingleton::~ISingleton() {}