ceph_test_rados_api_tier: make PromoteOn2ndRead test tolerate retries

If there is an ill-timed connection reset our read could get sent twice.
Weaken our assertion if the read was slow to tolerate this case.

Fixes: #9221
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2320 from ceph/wip-9221

ceph_test_rados_api_tier: make PromoteOn2ndRead test tolerate retries

Reviewed-by: Samuel Just <sam.just@inktank.com>