python: stop raising BadSignatures from decrypt(verify=True)

* src/core.py (decrypt): filter out signatures with errors from the
returned verify_result, but avoid raising BadSignatures
* tests/t-decrypt-verify.py: ensure that only a single signature is
returned when evaluating cipher-3.asc, since the other signature is
unknown.

This change preserves the invariant that decrypt() only ever returns
valid signatures in the verify_result, but it avoids unnecessary
errors in the face of the presence of an additional bad signature.

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