Wait for last form field to appear.

Hopefully this fixes "New user overlay closes on valid data"

RF test Jenkins currently fails:
http://jenkins.plone.org/job/plone-5.0-python-2.7/477/testReport/Products.CMFPlone.tests.test_robot/RobotTestCase/Scenario_New_user_overlay_closes_on_valid_data/)

Have reproduced  this locally, but not reliably.
What we saw locally was that email or username gets filled in as fullname, and
email stays empty. Maybe a timing issue.