Enable Examples in SimpleITK superbuild for testing

In the SimpleITK superbuild sub-project, examples are enabled when
testing is enabled. This is so that they are tested.

Change-Id: Ic7981e9326ef917c76b9e3ed2ac98dfcd15412c1
Merge topic 'SIMPLEITK-410_UseOptionBUILD_EXAMPLES' into next

4df5581 Enable Examples in SimpleITK superbuild for testing