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_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()`.