Fix compilation of tests against VTK6

This commit allows CTK to be compiler against VTK6 with testing enabled.
That said, there are some tests failing to pass. Issues #467, #468, #469
and #471 track the failure most likely related to VTK6.
Merge pull request #472 from jcfr/fix-test-compile-against-vtk6

Fix compilation of tests against VTK6