Fix flaky test
Fix flaky test

```
  1) Error:
DesktopMessagesTest#test_just_shows_a_special_label_when_the_interlocutor_is_no_longer_there:
ActiveRecord::RecordNotFound: Couldn't find User with 'id'=2
    app/controllers/conversations_controller.rb:28:in `create'
```

I think this was a thread syncronization issue. By waiting to the UI to
be there before destroying the user, I think we avoid it.

alabs/nolotiro.org,alabs/nolotiro.org,alabs/nolotiro.org,alabs/nolotiro.org