COMP: Fix usage of PYTHONPATH in extension launcher settings

Following r23036 environment variable specifying list of path are
handled differently. This commit ensures that python paths associated
with extension are also set consistently across Slicer application
and extension settings.

The additional extension launcher settings are used in macro
"slicer_add_python_test" and "slicer_add_python_unittest" implemented
in SlicerMacroPythonTesting.cmake module.

Since the macro "slicer_add_python_unittest" is used to configure the
generic tests from within the macro "slicerMacroBuildScriptedModule",
this commit should also help fix failing tests from extension bundling
scripted modules.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23530 3bd1e089-480b-0410-8dfb-8563597acbee