STYLE: For consistency, change CMake macro from "SIMPLE_TEST" to "simple_test"

Use of lower case CMake function/macro when including underscore has been
first introduced with r17191.

See http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=17191

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21704 3bd1e089-480b-0410-8dfb-8563597acbee