I don't know. I haven't seriously looked for one. There's Forge which seems to be still active but I have no idea whether you can use it like you want:
You'd need an AI player API, something that lets you hook an AI player into the engine and run it without graphics obviously so you can train fast. Forge has an AI player (or more?) but I don't know if can be used that way.
I guess with a bit of elbow grease you could make a bot to play Arena (and even play against itself), but that may not be acceptable by WotC.
