Bug 1049916 - Create Firefox Accounts smoke tests to validate user authe...

...ntication R=spenrose
Note that `data-l10n-ids` are not stable: due to the way our l10n system works, every time you change the string, you have to also change the data-l10n-id, or the localizers can't see the updated string. It's better to use an id, if possible. (Although I guess brittle descendant selectors like 'div span button label .foo' aren't much better...do what you can ;-)