reactjs + reflux // testing components fetching remote data

Pretty new to testing reacts apps (and JavaScript apps in general), I'd be looking into understanding how tools like mocha or others works, when you'd like to test a component that require a response from a remote server (meaning I want to verify the component has been populated with remote data, a user login has been confirmed, etc…)

So far I've looked into mocha docs, without understanding it very well — it won't just be using setTimeouts right ? Or is it transparent, the testing tool "understanding" it has to wait for the response out of the box ?