Handle warning detection in tests with pytest

[thomas-saigre]

Example in test_colors.py :

def test_color(color, warning_match, request):
    id = request.node.callspec.id
    if warning_match is None:
        assert_equality(plot_color(color), os.path.join(this_dir, test_name, f"{test_name}_{id}_reference.tex"))
    else:
        with pytest.warns(UserWarning, match=warning_match):
            assert_equality(plot_color(color), os.path.join(this_dir, test_name, f"{test_name}_{id}_reference.tex"))

Final goal is to have everything in green when pytest is run (for now we have some yellow !)