public async Task Differ_works()
        {
            var differ = new Differ(CONN_STR, KEY_PREFIX);

            // cache is empty. shouldn't be any diffs, but more importantly it shouldn't throw any errors
            (await differ.GetPreviousNotInCurrentAsync()).Should().BeEmpty();
            (await differ.GetCurrentNotInPreviousAsync()).Should().BeEmpty();

            // load day 1. all should be current not in previous, none vice-versa
            await differ.LoadCurrentAsync("day1", GetCsvDay1());

            (await differ.GetPreviousNotInCurrentAsync()).Should().BeEmpty();
            (await differ.GetCurrentNotInPreviousAsync()).Should().HaveCount(7);

            // load day 2. should return correct diffs compared to day 1
            await differ.LoadCurrentAsync("day2", GetCsvDay2());

            (await differ.GetPreviousNotInCurrentAsync())
            .Select(s => s.Substring(0, 4))
            .Should().BeEquivalentTo("0001", "0003", "0006");
            (await differ.GetCurrentNotInPreviousAsync())
            .Select(s => s.Substring(0, 4))
            .Should().BeEquivalentTo("0001", "0006", "0008");
        }