Bug 1083126 - Update test_sms_with_attachments.py to match the manual sm...

...oketest
Oops, I haven't seen that in my previous review. Per @davehunt email, this is the way used to wait for an element to be displayed and retain it for later use:

``` Python
element = Wait(self.marionette).until(expected.element_present(by, locator))
Wait(self.marionette).until(expected.element_displayed(element))
return element.text
```