tests: add two new types of encrypted data

* tests/gpg/cipher-3.asc: add an encrypted file containing signatures
(one from a known key, and one from an unknown key)
* tests/gpg/cipher-no-sig.asc: add an encrypted file containing no
signatures at all

We currently aren't using these files yet, but they should be useful
for exercising the library in unusual circumstances (where the bugs
are).

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>