Revert "Revert "Revert "Add ugly tearDown to fix tests since test-content sticks around."""

This reverts commit 6f1412e73258bd91b9e75219c319e2726b74a52b.
Use functional test layer instead of integration test layer. It seems that restrictedTraverse triggers the 'transaction commit' that has been introduced here: https://github.com/plone/Products.CMFPlone/commit/ffb3c7003dc138916a0ac24ad79bb222eea966a6#diff-61c34ef6e9878fc83769d1e409f1b668R295.