make xdmf3 test respect lack of hdf5 files
make xdmf3 test respect lack of hdf5 files

I vaguely recall that the old xdmf3 writer didn't respect the
cutover size limit and always made hdf5 files. In any case
the updated xdmf3 does and the test needs to respect that fact
that it is OK to have and xdmf file without an acoompanying
hdf5 file.

keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK