The docs have example responses, you can copy-paste those into your code and instead of actually firing the HTTP request you just put these into your return data.
Yes, it’s not elegant, but better than nothing, that way you can at least test/try your code that calls the API and does things with it.
For a “real life” test you’d then need to ask some streamers if they’d be willing to help you test it.
Let’s hope for that sandbox.