[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.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html