Set up plone.app.testing

Remove all traces of ``Products.PloneTestCase`` and
``Testing.ZopeTestCase`` from tests and instead use a
``plone.app.testing`` layer.

Notice that the only change to the tests (apart from
``setUp`` and ``tearDown``) is a change on
``test_fragment_skip_purge``. This change (upped a
counter by one) is due to using ``plone.app.testing``
which creates another viewletmanager (``Plone Default``)
by default.