qa/suites/upgrade/pacific-p2p: skip TestClsRbd.mirror_snapshot test

qa/suites/upgrade/pacific-p2p: skip TestClsRbd.mirror_snapshot test

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

    [ RUN      ] TestClsRbd.mirror_snapshot
    .../ceph-16.2.7/src/test/cls_rbd/test_cls_rbd.cc:2278: Failure
    Expected equality of these values:
      -85
      mirror_image_snapshot_unlink_peer(&ioctx, oid, 1, "peer2")
        Which is: 0
    [  FAILED  ] TestClsRbd.mirror_snapshot (30 ms)

TestClsRbd.snapshots_namespaces test was removed in commit 4ad9d565a15c
("librbd: simplified retrieving snapshots from image header") many years
ago.

Fixes: https://tracker.ceph.com/issues/62586
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>