Bolster test_settings_cell_data with an additional wait

This addresses the intermittent failure as seen at [1].

```all branches``` -> merge

[1] http://qa-selenium.mv.mozilla.com:8080/view/B2G%20Inari/job/b2g.inari.gaia.v1.0.1.ui.xfail.ril/213/testReport/junit/%28root%29/TestSettingsCellData/test_enable_cell_data_via_settings_app/?