Add Python test for subset_data called from command line

[samsrabin]

In #2081, I added an evenly_split_cropland option to the configuration files used by subset_data, but neglected to add it to the argument parser, meaning that it would fail when run from the command line. This wasn't caught by the Python tests we have set up because there's no test that uses subset_data with command-line arguments instead of a config file.

I wonder: Are there any other Python scripts with this same issue?

[samsrabin]

Note that @slevis-lmwg fixes the argument parser in #2253.