[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/lib/PY_VERSION/site-packages/cubicweb/devtools/ usr/lib/PY_VERSION/site-packages/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/skeleton/ usr/lib/PY_VERSION/site-packages/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/test usr/lib/PY_VERSION/site-packages/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/entities/test usr/lib/PY_VERSION/site-packages/cubicweb/entities/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/ext/test usr/lib/PY_VERSION/site-packages/cubicweb/ext/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/server/test usr/lib/PY_VERSION/site-packages/cubicweb/server/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/sobjects/test usr/lib/PY_VERSION/site-packages/cubicweb/sobjects/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/hooks/test usr/lib/PY_VERSION/site-packages/cubicweb/sobjects/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web/test usr/lib/PY_VERSION/site-packages/cubicweb/web/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/etwist/test usr/lib/PY_VERSION/site-packages/cubicweb/etwist/