Bug 1061215 - [Messages][Refactoring] Improve handling of remaining chars counter. r=schung

No, we can't. There are only few assert methods available in marionette js:
```
fail
ok
equal
notEqual
deepEqual
notDeepEqual
strictEqual
notStrictEqual
throws
doesNotThrow
ifError
```

So ```assert.equal(...., true/false)``` looks like the best option IMO.

> also, could it make sense to add a assert.isDisplayed with a meaningful error message?

Mmm, I'd rather leave assert methods as generic as possible