HeaderTesting: skip non-vtk* files

Change-Id: I5b6296dea51ff0bb872b7e3367aea7f750b40ae0
HeaderTests: only run if VTK_SOURCE_DIR is set

This isn't true for external modules.

Change-Id: Ie415816c4983c3319f73a8f5eb351a2a4eab27a5
Merge topic 'header-test-improvements' into master

e48e1100 HeaderTests: only run if VTK_SOURCE_DIR is set
2247d891 HeaderTesting: skip non-vtk* files