Check runoff mapping for multi-ice sheet cases

We're doing our initial development of Antarctica and multiple ice sheets with T and I compsets. A next step will be to check the glc -> ocn mapping in a B compset.

After generating runoff mapping files for Antarctica (https://github.com/ESMCI/cime/issues/4097), we should check:
- [ ] Run a short B compset with Antarctica with test_coupling: Make sure the glc to ocn mapping looks reasonable. (This is to confirm that Antarctica and its related mapping files are set up properly to send runoff to the ocean. We may want to zero out other runoff or set glc's runoff to huge in order to be able to see this more clearly – i.e., so other land runoff doesn't contaminate the signal that we're looking for.)
- [ ] Run a short B compset with both Greenland and Antarctica. The glc to ocean runoff over Greenland should pretty closely match that from a Greenland-only B case; and the glc to ocean runoff over Antarctica should pretty closely match that from an Antarctica-only B case. To check this, we should check the runoff that has been remapped to the ocean grid. As above, it may help to zero out other runoff sources for this test. The main point of this is to make sure that CMEPS is handling the multiple glc -> ocn runoff mapping files correctly.

**Update 2024-06-05: see new plan in https://github.com/ESCOMP/CISM-wrapper/issues/64#issuecomment-2150664137**