rbd: update test case for "old" deprecated open
rbd: update test case for "old" deprecated open

Rename test case to mark the "old" deprecated open func as such.
This also adds a missing bit of coverage to the test.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph