Bug 943165 - [Messages] Bug 934531 follow-up: make the visual right for ...

...numbers-only headers r=julien
sorry, the line is now too long according to our style checker.
You can do something like:
```js
  assert.ok(
    calls[0].header.classList.contains('unknown-contact-header')
  );
```

(note that we try to always go to a new line with an additional 2-space indentation, we don't try to align text)