When GnuPG refuses to encrypt a message (e.g. when key has expired), record
information about the failure and send to logs, then deliver cleartext. This
way we won't bounce email that could be delivered without encryption.
Also: add more E2E tests.
- Cover basic GnuPG functionality with tests (confirm_key, public_keys).
- Add a test public key file.
- Fix resource leak by closing streams opened by Popen.
In particular, move:
- test utilities to test/utils;
- unit tests to test/modules.
Also: start implementing the Lacre daemon test (just a stub for now).