And now I see clearly that we don't mock mozL10n in our tests and therefore we probably get unreliable results, because our tests will break if the localization change.

The proper way to do this now could be to:
* have a suiteSetup function in the top level suite, with a `done` argument
* in this suiteSetup, setup a mozl10n mock (you can see how it's done in the system app)
* only then, requireApp('sms/js/utils.js', done);

I'd be happy enough that it happens in another pull request though.