tests/test_util.py: Add tests for `cwd()`, `natural_sort_path()`, `system()`, `absolute_path()`, `transform_fd_to_tmpfile()`, `get_cert_paths()`, `get_parent_cert_paths()` and `contains_fingerprint()`.
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()`.
tests/test_git.py: Add unit test for `git_changed_files()`.
tests/*: The existing coverage configuration is setup to use/ omit a tests directory.
