Replace `data` initialization in duo_chat.spec.js

Context

The following discussion from !4017 (merged) should be addressed:

• @pslaughter started a discussion:

  polish (non-blocking): We usually avoid setting data either with setData or with the data prop of shallowMount since this isn't actually an input into the component. We can take care of this in a follow-up. I'll create the issue 👀