Add a wait for intermittent failure of test_edit_contact

This is intended to address the intermittent failure of test_edit_contact, which can be seen at http://qa-selenium.mv.mozilla.com:8080/view/B2G/job/b2g.unagi.gaia.master.ui/1265/testReport/%28root%29/TestContacts/test_edit_contact/, and I have also seen locally.