qa/suites/upgrade/octopus-x: skip TestClsRbd.mirror_snapshot test

qa/suites/upgrade/octopus-x: skip TestClsRbd.mirror_snapshot test

The behavior of the class method changed in reef; the change was
backported to pacific and quincy.  An octopus test binary used against
quincy OSDs produces an expected failure:

    [ RUN      ] TestClsRbd.mirror_snapshot
    .../15.2.17/rpm/el8/BUILD/ceph-15.2.17/src/test/cls_rbd/test_cls_rbd.cc:2279: Failure
    Expected equality of these values:
      -85
      mirror_image_snapshot_unlink_peer(&ioctx, oid, 1, "peer2")
        Which is: 0
    [  FAILED  ] TestClsRbd.mirror_snapshot (6 ms)

Fixes: https://tracker.ceph.com/issues/62402
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>