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

...oketest
The save button is accessible [with an ID](https://github.com/mozilla-b2g/gaia/blob/master/apps/gallery/js/open.js#L274). I would replace these 2 lines with `self.marionette.find_element(By.ID, 'save').tap()`