Install Products.ATContentTypes on the test fixture because Plone 5 does not install ATContentTypes automatically any longer.
Merge branch 'master' into mockup-issue-220

* master:
  Install Products.ATContentTypes on the test fixture because Plone 5 does not install ATContentTypes automatically any longer.
Revert "Install Products.ATContentTypes on the test fixture because Plone 5 does not install ATContentTypes automatically any longer."

This reverts commit ea6b44462fad113fcc4922542d614fc3240bb927.
Merge branch 'master' into mockup-issue-220

* master:
  Revert "Install Products.ATContentTypes on the test fixture because Plone 5 does not install ATContentTypes automatically any longer."