[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.
# copyright %(year)s %(author)s, all rights reserved.# contact %(author-web-site)s -- mailto:%(author-email)s#%(long-license)s"""%(distname)s precreate script, executed at instance creation time or whenthe cube is added to an existing instance, before the schema is serialized.This is typically to create groups referenced by the cube'schema."""