Add RXCROPMATURITY test to aux_clm

As discussed earlier with @ekluzek, I think it'd be good to have the RXCROPMATURITY system test run as part of the aux_clm test suite. However, in its usual configuration—SMS_Ly5 on the 10x15 global grid—it needs a walltime of 3 hours, since it runs the simulation twice and does some processing in between. This is as long as the longest existing test.

How short would I need to make it for it to be palatable as part of aux_clm?

Some ideas for shortening it:
- Reducing from 5 years to 4. This would mean that only one year is actually checked in my Python scripts, which is probably fine.
- Using a custom 10x15 grid that only includes every other latitude row. There are some edge cases that happen at high latitudes, and I also want to be sure to test tropical CFTs in their proper zone. (So I can't just say "high latitudes" or "tropics.")
- Using the PPE grid?