Use vtk_test_cxx_executable in Rendering/GL2PS/Testing/Cxx

Replace the last use of vtkTestingObjectFactory.cmake and remove it.
CMake modules that have immediate inline effects are hard to follow, so
drop this one.

Change-Id: I813644b0c31c1eecd3415f11e57e703d035940fd
Merge topic 'cleanup-vtkTestingObjectFactory-cmake' into master

57569cc9 Use vtk_test_cxx_executable in Rendering/GL2PS/Testing/Cxx