Lower testing of RTM in CESM3

[ekluzek]

The use of RTM is now only for Paleo work in CESM3. As such we should lower the testing for it, and only test what's needed for Paleo work in CESM3. In CESM4 we hope to move to mizuRoute and hence remove RTM in CESM4.

This relates to #42

[ekluzek]

To facilitate this let's remove the flooding tests which is available in MOSART, and remove izumi as a machine to test on. So testing will just be done on Derecho for test list that is very short.

[ekluzek]

I was wrong in the above statement, flooding is NOT available nor working in MOSART. See

ESCOMP/MOSART#80

As such we should keep the flooding tests in RTM so we ensure passing the fields through the CPL continues to work. We do want a working version in MOSART, but until then we should continue to test in RTM.

Note, also that CTSM runs some flooding tests in aux_clm:

ERP_P64x2_D.f10_f10_mg37.I2000Clm50SpRtmFl.derecho_intel.clm-default
ERS_D_Ld5.f10_f10_mg37.I2000Clm50BgcCropRtm.derecho_intel.rtm-rtmOnFloodOnEffvelOn

So the RTM test list should continue to test those.