BUG: Fix regression: python code "exit(status)" will now exit with "status"

This commit fixes py_nomainwindow_SlicerPythonCodeTest2 fixing a regression
introduced in r25083 (STYLE: Automatically enable "ignore-slicerrc" if
"testing" option is given).

Associating the test with "--testing" option was causing the application
to always exit with EXIT_SUCCESS when "exit(EXIT_FAILURE)" was called.

This commit ensures the all to slicer.app.exit() in "slicer.util.exit()"
is considered.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25100 3bd1e089-480b-0410-8dfb-8563597acbee