[test/windmill] Gracefully skip windmill test if windmill can't be imported.
If windmill import fails the windmill module is set to None and a dummy
CubicWebWindmillUseCase test class is create. This class contains a test that
skip itself with a proper warning message.
#! /bin/sh -e
if [ -x "/etc/init.d/cubicweb-ctl" ]; then
invoke-rc.d cubicweb-ctl restart || true
fi
#DEBHELPER#
exit 0