Improve directory.TestGetPutSignatures
Improve directory.TestGetPutSignatures

- Use a different manifest and manifest list
- Write per-arch manifest before the top-level one
- Same for signatures
- Don't write the top-level manifest twice

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image