ENH: Unit test for ParametricSpline

Provide 100 % coverage in a unit test for vtkParametricSpline.
This illustrates it can take considerable effort to provide 100%
coverage.

Change-Id: I31e9a7bc4b6e31cb23209630520d01980d0a09c9
Merge topic 'Coverage_ParametricSpline' into master

e2b5f16b ENH: Unit test for ParametricSpline