Ported tests to plone.app.testing.

This PR ports the tests of p.a.versioningbehavior to p.a.testing, based on the changes in https://github.com/plone/Products.CMFEditions/pull/19 which does the same for CMFEditions.

This requires Plone 5 and the branch of the [CMFEditions pull-request](https://github.com/plone/Products.CMFEditions/pull/19).

/cc @tomgross