Let the @browsing decorate work on unit tests too.

If the test has no layer, the @browsing can still be used now.
However it only works when loading pages with `open_html`, since
there is no Plone or Zope app to connect to.
Add support for filling AT MultiSelectionWidget.
Merge pull request #12 from 4teamwork/jone-multiselect-widget

Add support for filling AT MultiSelectionWidget.