subset_data calls in mksurfdata_esmf Makefile are missed by Python testing

I did the complete Python testing when I brought in #3002, but I didn't think to test `mksurfdata_esmf`. It turns out #3002 broke the `subset_data` call for `1x1_brazil` in that tool's Makefile. I've fixed this in #2737, but we should make it so that the standard Python testing includes the `mksurfdata_esmf` calls. (Just the calls of `subset_data`; we don't need to actually build the datasets.)

Possible resolutions:
1. Add an option to the Makefile that allows everything to be skipped except the `subset_data` calls, then add a Python system test that just builds the tool and then does `make all`.
2. Replace the Makefile with Python code, then do the same thing. This would also allow us to add _unit_ tests for the standard dataset generation process.