Add wait to bolster test_dialer_add_contact

This addresses the failure as seen at http://qa-selenium.mv.mozilla.com:8080/view/B2G%20Unagi/job/b2g.unagi.mozril.gaia.master.ui/lastCompletedBuild/testReport/%28root%29/TestDialerAddContact/test_dialer_add_contact/history/?

`master` -> merge
`v1-train` -> test and merge