Method async_callback

fs/test/test.cpp:560–563

    {
    public:
        int async_callback(AsyncResult<ssize_t>* ar)
        {
            return ::async_callback(this, ar);
        }
        void do_test(IAsyncFile* f)
        {
            f->pread(nullptr, 200, 300, {this, &TestAsyncFS::async_callback});