[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.

msgid ""
msgstr ""
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"Generated-By: pygettext.py 1.5\n"
"Plural-Forms: nplurals=2; plural=(n > 1);\n"