test is currently flaky since system reads can happen
in the background and disturb the global row cache stats.
Use the table's row_cache stats instead.
Fixes #6773
Test: cql_query_test.test_cache_bypass(dev, debug)
Credit-to: Botond Dénes <bdenes@scylladb.com>
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Message-Id: <20200811140521.421813-1-bhalevy@scylladb.com>
(cherry picked from commit 6deba1d0b4)