Added missing private dependency, disabled test

Adding missing private dependency on vtkIOImage for vtkRenderingParallel
and disabled a Python test that is equivalent to the C++ tests using XML
shaders to test generic attributes.

Change-Id: I4a29650fc8aeb0cbc2800aed8daab5a695cabc9c
Merge topic 'rendering-remove-xml' into master

980e8951 Added missing private dependency, disabled test