For testing in chat, you can go to literally any channel on Twitch that’s an affiliate or partner and either cheer yourself or just wait for a cheer from someone else. You don’t need to test on your own test channel. Or you could just create mock data based on the documented specifications and send that to your app to test.
For testing in EventSub, the Twitch CLI includes being able to trigger EventSub notifications https://dev.twitch.tv/docs/cli