Add direct tests for CLI

[rhugonnet]

Working on #863, I notice that we only have Python tests for Workflows right now, no direct CLI tests.

We should inspire ourselves from the CLI tests in GeoUtils here: https://github.com/GlacioHack/geoutils/blob/main/tests/test_geoviewer.py and add a test_cli.py test suite.

Among others, this would allow to check that the users do get the right error message when optional dependencies are missing (what I'm working on), and that the CLI does raise standard errors (what I mentioned in #858).