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.
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.