ENH: Added a test for the ExtractSkeleton CLI. Fixes #3333

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Dirk Padfield <padfield@ge.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23657 3bd1e089-480b-0410-8dfb-8563597acbee