Fix false positive on res_stats checking in cache unit test

        EXPECT_TRUE(params == expected.setup.params) << msg;

        // res_stats checking.

        if (expected.stats.size() == 0) {

            for (int ns = 0; ns < nscount; ns++) {

                EXPECT_EQ(0U, stats[ns].sample_count) << msg;

            }

        }

        for (size_t i = 0; i < expected.stats.size(); i++) {

            EXPECT_TRUE(stats[i] == expected.stats[i]) << msg;

        }