qa: test devicehealth legacy load of deleted snap obj

qa: test devicehealth legacy load of deleted snap obj

Failure without fix looks like:

    2023-12-21T16:05:55.737+0000 7fbe585b0700  0 [devicehealth DEBUG root] loading object ABC_DEADB33F_FA
    2023-12-21T16:05:55.737+0000 7fbe585b0700 -1 log_channel(cluster) log [ERR] : Unhandled exception from module 'devicehealth' while running on mgr.x: [errno 2] RADOS object not found (Failed to operate read op for oid ABC_DEADB33F_FA)
    2023-12-21T16:05:55.737+0000 7fbe585b0700 -1 devicehealth.serve:
    2023-12-21T16:05:55.737+0000 7fbe585b0700 -1 Traceback (most recent call last):
    File "/home/pdonnell/ceph/src/pybind/mgr/devicehealth/module.py", line 394, in serve
        self._do_serve()
    File "/home/pdonnell/ceph/src/pybind/mgr/mgr_module.py", line 524, in check
        return func(self, *args, **kwargs)
    File "/home/pdonnell/ceph/src/pybind/mgr/devicehealth/module.py", line 354, in _do_serve
        finished_loading_legacy = self.check_legacy_pool()
    File "/home/pdonnell/ceph/src/pybind/mgr/devicehealth/module.py", line 326, in check_legacy_pool
        if self._load_legacy_object(ioctx, obj.key):
    File "/home/pdonnell/ceph/src/pybind/mgr/devicehealth/module.py", line 300, in _load_legacy_object
        ioctx.operate_read_op(op, oid)
    File "rados.pyx", line 3723, in rados.Ioctx.operate_read_op
    rados.ObjectNotFound: [errno 2] RADOS object not found (Failed to operate read op for oid ABC_DEADB33F_FA)

Credit to Greg Farnum for postulating the cause.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit aa30adbaa7616c8e24b3ccaad9dbcda7c0c663fc)