I’m currently trying to start a project but it requires me to be able to interact with broadcaster accounts that have subscribers, and user accounts that are subscribed to streams.
Is there a sandbox environment that lets me create dummy accounts to work with?
Not currently but it would be nice. 
The example responses on Github should be accurate though.
I’m definitely keen for Twitch to create a sandbox environment. There are various nuances in the way the API behaves that aren’t documented and need to be discovered (e.g. the offset limit on the followers endpoint).