Fix for SimuJobTransforms/test_diffmerged.sh ART job (ATLASSIM-4198)
In test_diffmerged.sh
we were creating a HITS file, then copying it using mergePOOL
then merging the original and the copy into a new HITS file. This no longer works in
master, so have switched to running two simulation jobs instead.
Should fix ATLASSIM-4198.