test/rbd: add test for opening an Image read-only
test/rbd: add test for opening an Image read-only

Signed-off-by: Niels de Vos <7bda0c0ee7f47171cc808e9e60292cbc5d8140dd@redhat.com>

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