Add a robotframework test for news item migration. Factor out a new noin...

...stall test fixture that doesn't install p.a.contenttypes in portal_quickinstaller.

I expected the test to break on edit of the news item after the migration, but it doesn't.
@pbauer did I miss something?

Also, I'm not sure this test was intended to go to master, but I cant' ask (or don't know how to) for a new branch in a pull request.