Bug 968677 - [settings] add marionette test for Messaging Settings panel

For the consistency[1], could we remove this method. We could just do
```
get deliveryReportCheckboxChecked() {
  return !!this.findElement('deliveryReportCheckbox').getAttribute('checked');
}
...
```
here.
[1] https://github.com/mozilla-b2g/gaia/blob/master/apps/settings/test/marionette/app/regions/root.js#L74