tests/test_keyring.py:
Add tests for `is_pgp_fingerprint()`,
`transform_{fingerprint,username}_to_keyring_path()`,
`derive_username_from_fingerprint()` and get_fingerprints_from_paths()`.
tests/conftest.py:
Add `valid_fingerprint()` and `valid_subkey_fingerprint()` fixtures to
produce a generic "valid" PGP fingerprint string.
Add the `invalid_fingerprint()` fixture to generate a set of "invalid"
fingerprint strings.
CalledProcessError returns bytes for our invocations, the fix that
decoded bytes of stdout was purely to make the mocked test happy while
breaking the actual usage. Restore the behavior and fix the wrong mocked
data.
tests/test_sequoia.py:
Add unit tests for `keyring_split()`, `keyring_merge()`,
`packet_split()`, `packet_join()`, `inspect()`, `packet_dump()`,
`packet_dump_field()`, `packet_signature_creation_time()`, and
`latest_certification()`.
