This commit adds a new test file, `tests/test_cli.py`, containing
unit tests for the `_os_timezone` function in `netsleuth.cli`.
The tests cover:
- Happy paths (TZ variable, /etc/localtime symlink, /etc/timezone file)
- The edge case of an `OSError` exception being thrown when reading the
/etc/timezone file.
Co-authored-by: Horizontal42 <75383519+Horizontal42@users.noreply.github.com>
