Add a wait in test_sms to address intermittent failures

Although it doesn't seem like it should be necessary, I can see failures of test_sms when the phone number field isn't visible, e.g., [1]. I have also seen this locally. Adding the wait in this pull request seems to eliminate the problem.

`master` --> merge
`v1-train` --> merge

[1] http://qa-selenium.mv.mozilla.com:8080/view/B2G%20Unagi/job/b2g.unagi.mozril.gaia.v1-train.ui/597/testReport/junit/%28root%29/TestSms/?