FastCaloSim in G4: Drop diff-pool checks for MT jobs

diff-pool checks can show differences due to the different ordering of events in the output file. This is nothing to be concerned out, so disabling them for test_AtlasG4_FastCaloSim_MT.sh and test_AtlasG4_FastCaloSim_MTvsST.sh Essentially a backport of !69612 (merged) to 24.0

Should fix ATLASSIM-7204

Simulation/Tests/SimCoreTestsMT/test/test_AtlasG4_FastCaloSim_MT.sh

@@ -38,7 +38,7 @@ if [ $rc -eq 0 ]
 then
     ArtPackage=$1
     ArtJobName=$2
-    art.py compare grid --entries 20 ${ArtPackage} ${ArtJobName} --mode=semi-detailed --order-trees
+    art.py compare grid --entries 20 ${ArtPackage} ${ArtJobName} --mode=semi-detailed --order-trees --diff-root
     rc2=$?
     if [ $status -eq 0 ]
     then