Change LGRAIN tests to 1850

Ideally we would do year-2000 tests to have more crop cover and thus
potentially be more useful tests. However, there are problems running a
year-2000 ciso test with crop. These problems exist even with an SMS
test on master:

I tried tests like
SMS_Ly1_P72x1.f10_f10_mg37.I2000Clm45BgcCrop.cheyenne_gnu.clm-ciso--clm-cropMonthOutput,
but both debug & non-debug, intel & gnu versions.

Debug tests fail like this (from SMS_D_Ly1_P72x1.f10_f10_mg37.I2000Clm45BgcCrop.cheyenne_gnu.clm-ciso--clm-cropMonthOutput):

30:Program received signal SIGFPE: Floating-point exception - erroneous arithmetic operation.
30:
30:Backtrace for this error:
13:
13:Program received signal SIGFPE: Floating-point exception - erroneous arithmetic operation.
13:
13:Backtrace for this error:
13:#0  0x2b9d1acc4aff in ???
30:#0  0x2b9d1acc4aff in ???
13:#1  0xf63fff in cisofluxcalc
13:     at /glade/work/sacks/ctsm_code/ctsm/src/biogeochem/CNCIsoFluxMod.F90:1555
30:#1  0xf63fff in cisofluxcalc
30:     at /glade/work/sacks/ctsm_code/ctsm/src/biogeochem/CNCIsoFluxMod.F90:1555
30:#2  0xf6b489 in __cncisofluxmod_MOD_cisoflux1
30:     at /glade/work/sacks/ctsm_code/ctsm/src/biogeochem/CNCIsoFluxMod.F90:153
13:#2  0xf6b489 in __cncisofluxmod_MOD_cisoflux1
13:     at /glade/work/sacks/ctsm_code/ctsm/src/biogeochem/CNCIsoFluxMod.F90:153
13:#3  0xe45657 in __cndrivermod_MOD_cndrivernoleaching
13:     at /glade/work/sacks/ctsm_code/ctsm/src/biogeochem/CNDriverMod.F90:559
30:#3  0xe45657 in __cndrivermod_MOD_cndrivernoleaching
30:     at /glade/work/sacks/ctsm_code/ctsm/src/biogeochem/CNDriverMod.F90:559

An intel test dies in the same place.

Non-debug versions die like this (both for gnu and intel):

30: set_curr_delta ERROR: found unexpected non-zero delta mid-year
30: Dribbler name: hrv_xsmrpool_to_atm_c_13
30: i, delta =            2                       NaN
30: Start of time step date (yr, mon, day, tod) =         2000           1          15       57600
30: This indicates that some non-zero flux was generated at a time step
30: other than the first time step of the year, which this dribbler was told not to expect.
30: If this non-zero mid-year delta is expected, then you can suppress this error
30: by setting allows_non_annual_delta to .true. when constructing this dribbler.
30:iam = 30: local  gridcell index = 2
30:iam = 30: global gridcell index = 103
30:iam = 30: gridcell longitude    =  285.0000000
30:iam = 30: gridcell latitude     =  -10.0000000
30: ENDRUN:
30: ERROR: set_curr_delta: found unexpected non-zero delta mid-year: ERROR in /glade/work/sacks/ctsm_code/ctsm/src/utils/AnnualFluxDr
ibbler.F90 at line 276

So there is some issue with year-2000 ciso tests with crop. This issue
exists on master, for clm45 and clm50 tests. (e.g., for clm50, I tried
SMS_D_Ly1_P72x1.f10_f10_mg37.I2000Clm50BgcCrop.cheyenne_gnu.clm-ciso--clm-cropMonthOutput.)

cime_config/testdefs/testlist_clm.xml

@@ -2498,7 +2498,7 @@
     </machines>
   </test>
 
-  <test name="LGRAIN2_Ly1_P72x1" grid="f10_f10_mg37" compset="I2000Clm50BgcCrop" testmods="clm/ciso--clm/cropMonthOutput">
+  <test name="LGRAIN2_Ly1_P72x1" grid="f10_f10_mg37" compset="I1850Clm50BgcCrop" testmods="clm/ciso--clm/cropMonthOutput">
     <machines>
       <machine name="cheyenne" compiler="gnu" category="aux_clm">
         <options>
@@ -2508,7 +2508,7 @@
       </machine>
     </machines>
   </test>
-  <test name="LGRAIN2_Ly1_P72x1" grid="f10_f10_mg37" compset="I2000Clm45BgcCrop" testmods="clm/ciso--clm/cropMonthOutput">
+  <test name="LGRAIN2_Ly1_P72x1" grid="f10_f10_mg37" compset="I1850Clm45BgcCrop" testmods="clm/ciso--clm/cropMonthOutput">
     <machines>
       <machine name="cheyenne" compiler="gnu" category="aux_clm">
         <options>