python/tests: try to decrypt and verify new test data

* lang/python/tests/t-decrypt.py: test decryption of cipher-3.asc and
cipher-no-sig.asc
* lang/python/tests/t-decrypt-verify.py: test decryption and
verification of cipher-3.asc and cipher-no-sig.asc

note that this introduces a failed test -- decrypt-verify.py
misbehaves on cipher-3.asc by throwing a BadSignature even though

• GnuPG-bug-id: T4276
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>