add test_object_copy_16m to test refcounting of tail objects
add test_object_copy_16m to test refcounting of tail objects

i don't think any of our CopyObj test cases were large enough to have
tail objects, so weren't exercising our tail object ref counting
strategy

Signed-off-by: Casey Bodley <2544ff357e70e8520f24ff4e53fde352cbf7d2cf@redhat.com>

ceph/s3-tests,ceph/s3-tests