DOC: CreateTestDriver srcs should have the .cxx extension.

CMake can have a hard time finding the relative path without them.
The extensions are used for the ITK tests drivers.

Change-Id: I1cd1cb95fdb484d732540a77755f61cc7ca88bfc
Merge topic 'TestDriverDoc'

8fa6806 DOC: CreateTestDriver srcs should have the .cxx extension.