Add aux_clm test that generates history_fields_fates.rst

The `clm/extra_outputs` testmod makes it so that either `history_fields_nofates.rst` or `history_fields_fates.rst` is generated, depending on whether it's a FATES compset. There is one test in aux_clm doing this with a non-FATES compset, but none with FATES. This means that if we want to update the history fields files in the docs, we have to manually run a FATES compset.

I think the easiest thing to do would be to just add `--clm/extra_outputs` to the `testmods` tag of an existing FATES test.