rgw_file: fix mv/rename cases broken by zipper integration

rgw_file: fix mv/rename cases broken by zipper integration

There were two problems.  First, leaf object names must be
expressed as fully-qualified to the bucket as input to the
copy-object step.  Second, handle s->object in the same step
indicates the being-created destination object of the copy,
this was correct in the original zipper change but broken
later.

* add a rename/mv unit test

Tests for the following cases added:

1. move between two sub-directory paths in a single bucket
2. move between two names at the top level of a single bucket
3. move between sub-directory paths in different buckets (cross-bucket rename)

Fixes: https://tracker.ceph.com/issues/64950
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>