[test] test_ff_stokes_channel_neumann_x_dirichlet_y sometimes fails

with the following error:

Fuzzy comparison...
 Traceback (most recent call last):
   File "/data/src/dumux/bin/testing/runtest.py", line 63, in 
     result = compare_vtk(args['files'][i*2], args['files'][(i*2)+1], relative=args['relative'], absolute=args['absolute'], zeroValueThreshold=args['zeroThreshold'])
   File "/data/src/dumux/bin/testing/fuzzycomparevtu.py", line 52, in compare_vtk
     root2 = ET.fromstring(open(vtk2).read())
   File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1312, in XML
     return parser.close()
   File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1671, in close
     self._raiseerror(v)
   File "/usr/lib/python2.7/xml/etree/ElementTree.py", line 1523, in _raiseerror
     raise err
 xml.etree.ElementTree.ParseError: no element found: line 466, column 16
 Test #11: test_ff_stokes_channel_neumann_x_dirichlet_y ............***Failed    5.80 sec

there seems to be something wrong with the vtu.