[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.

debian/tmp/usr/bin/cubicweb-ctl usr/bin/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/cwctl.py usr/lib/PY_VERSION/site-packages/cubicweb
debian/cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl