Fix python test failure because of MPI and vtkNrrdReader

The object factory overrides vtkNrrdReader to vtkPNrrdReader when an MPI
build is enabled. Since, the test is not added as a parallel test,
it is run with `vtkpython` instead of `pvtkpython` resulting in an
error.

This change disables the object factory override for vtkNrrdReader.

Rendering/Volume/Testing/Python/TestGPURayCastUserShader.py

@@ -29,6 +29,9 @@ VTK_DATA_ROOT = vtkGetDataRoot()
 '''
 sys.dont_write_bytecode = True
 
+# Disable object factory override for vtkNrrdReader
+vtk.vtkObjectFactory.SetAllEnableFlags(False, "vtkNrrdReader", "vtkPNrrdReader")
+
 dataRoot = vtkGetDataRoot()
 reader = vtk.vtkNrrdReader()
 reader.SetFileName("" + str(dataRoot) + "/Data/tooth.nhdr")