Method getInternal

tests/catch.hpp:9866–9873

    class Singleton : SingletonImplT, public ISingleton {

        static auto getInternal() -> Singleton* {
            static Singleton* s_instance = nullptr;
            if( !s_instance ) {
                s_instance = new Singleton;
                addSingleton( s_instance );
            }
            return s_instance;
        }

    public:
        static auto get() -> InterfaceT const& {