Temporarily change nuopc test to not use threading

I'm temporarily changing the processor layout for
SMS_D_Ln9_P480x3_Vnuopc.f19_g17.IHistClm50Sp.cheyenne_intel.clm-waccmx_offline
from 480x3 to 480x1 because it is sometimes hanging with a 480x3 layout.
Once we have resolved the threading-related hangs with nuopc documented
in ESCOMP#1331, we can change this back
to use threading.

cime_config/testdefs/testlist_clm.xml

@@ -209,7 +209,11 @@
       <option name="comment">Run a transient case with standalone settings similar to the FXHIST waccm test</option>
     </options>
   </test>
-  <test name="SMS_D_Ln9_P480x3_Vnuopc" grid="f19_g17" compset="IHistClm50Sp" testmods="clm/waccmx_offline">
+  <test name="SMS_D_Ln9_P480x1_Vnuopc" grid="f19_g17" compset="IHistClm50Sp" testmods="clm/waccmx_offline">
+    <!-- BUG(wjs, 2021-04-10, ESCOMP/CTSM#1331) I'm changing the processor layout for this
+         test from 480x3 to 480x1 because it is sometimes hanging with a 480x3 layout.
+         Once we have resolved these threading-related hangs with nuopc, we can change
+         this back to use threading. -->
     <machines>
       <machine name="cheyenne" compiler="intel" category="aux_clm"/>
     </machines>