test-oci.sh: Remove confusing references to 'registry'
test-oci.sh: Remove confusing references to 'registry'

The directory where we export an OCI image to is not a "registry", even
if it's a FlatpakOciRegistry internally. So change export registry from
oci/registry to oci/image.

Closes: #1910
Approved by: alexlarsson

flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak