Use VTK_TEST_OUTPUT_DIR rather than rebuilding the path

All of the other test variants use this variable; MPI should as well.

Change-Id: I7d67cabf697cf6bd2e40a0e70d66d16d26766464

CMake/vtkTestingMacros.cmake

@@ -119,7 +119,7 @@ function(vtk_add_test_mpi exename tests)
     set(_V "")
     if(local_TESTING_DATA)
       set(_D -D ${data_dir})
-      set(_T -T ${VTK_BINARY_DIR}/Testing/Temporary)
+      set(_T -T ${VTK_TEST_OUTPUT_DIR})
       if(local_CUSTOM_BASELINES)
         set(_V -V "${data_dir}/Baseline")
       else()