Add utils, CLI and config tests

[polyedr]

This PR adds a small set of basic tests to improve reliability:

• utils: ensure save_videos_from_pil writes a valid GIF file
• utils: ensure download_files_from_repo triggers snapshot_download when needed
• CLI: check that "python -m tango --help" runs successfully and prints usage
• config: verify that a simple YAML config can be loaded without errors

These tests are simple, fast, and help catch regressions early.
No functional changes to the codebase.

These tests are written using pytest.
To run them locally:

pip install pytest
python -m pytest -q