Fix generic test and reorg cmakelist

This is a follow-up of http://slicer-devel.65872.n3.nabble.com/Failure-to-load-the-extension-widget-from-generic-test-and-self-test-tt4029939.html


Slicer >= [r22430](http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=22430) is needed to test these changes.