fix up voidfractionTest case [ci skip]

add post folder and testharness configuration

tutorials/cfdemSolverPiso/voidfractionTest/postrun.sh

@@ -0,0 +1,2 @@
+#!/bin/bash
+# nothing to see here

tutorials/cfdemSolverPiso/voidfractionTest/prerun.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+#- define variables
+casePath="$(dirname "$(readlink -f ${BASH_SOURCE[0]})")"
+
+# check if mesh was built
+if [ -f "$casePath/CFD/constant/polyMesh/boundary" ]; then
+    echo "mesh was built before - using old mesh"
+else
+    echo "mesh needs to be built"
+    cd $casePath/CFD
+    blockMesh
+fi

tutorials/cfdemSolverPiso/voidfractionTest/run.config

@@ -0,0 +1,13 @@
+{
+  "type" : "CFDEMcoupling",
+  "runs" : [
+    {
+      "name" : "cfdemrun",
+      "solver" : "cfdemSolverPiso",
+      "type" : "CFDEMcoupling/mpi",
+      "nprocs" : 2,
+      "pre_scripts" : ["prerun.sh"],
+      "post_scripts" : ["postrun.sh"]
+    }
+  ]
+}