Method fetch_add

tests/framework/src/doctest.cpp:366–368

        T operator++(int) DOCTEST_NOEXCEPT { return fetch_add(1); }

        T fetch_add(T arg, std::memory_order order = std::memory_order_seq_cst) DOCTEST_NOEXCEPT {
            return myAtomic().fetch_add(arg, order);
        }

        T fetch_sub(T arg, std::memory_order order = std::memory_order_seq_cst) DOCTEST_NOEXCEPT {
            return myAtomic().fetch_sub(arg, order);