test_exception_safety_of_transitioning_from_underlying_read_to_read_from_cache: use flat reader
test_exception_safety_of_transitioning_from_underlying_read_to_read_from_cache: use flat reader

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

scylladb/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla