test/librbd: make rollback in TestGroup.add_snapshot{,PP} meaningful

test/librbd: make rollback in TestGroup.add_snapshot{,PP} meaningful

The rollback assert doesn't really test anything -- because orig_data
and test_data are written to non-overlapping areas, the test would pass
even if rbd_group_snap_rollback() does nothing (i.e. rollback isn't
performed) as long as the call returns 0.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit b3977c53c930100310868096408ad059dc5cad9b)