BUG: Fix vtkMRMLNonlinearTransformNodeTest1

This commit fixes the reading of ITKv3 transforms by addressing
a regression introduced in r24875 (BUG: Fix transform tests by not
using dynamic_cast in vtkITKTransformConverter)

It basically ensures that executable created using compiler (e.g g++4.4.3)
[not supporting the use of dynamic_cast with templated classes instantiated
in a translation unit different than the one where they are defined] still
work as expected.

See 0004139

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25318 3bd1e089-480b-0410-8dfb-8563597acbee