BUG: Update SubjectHierarchyGenericSelfTestTest to support VTK6. Fixes #3720

In both VTK 5 and 6 cases, the pipeline is connected and the
explicit call to "Update()" is not required.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23255 3bd1e089-480b-0410-8dfb-8563597acbee