How to cancel Auto-renewable subscription by using test user in sandbox environment?

You can do it in the following way:

1. Open iOS Settings
2. Go to “App Store”
3. Tap your sandbox account (“SANDBOX ACCOUNT” section at the bottom of the screen)
4. Select “Manage”
5. “Edit subscription” screen will open and there you’ll have a “Cancel subscription” option for your Sandbox subscription.

With a small delay (it was several minutes for me) you’ll get a server notification with "notification_type": "DID_CHANGE_RENEWAL_STATUS" and "auto_renew_status": "false". This means that the subscription is still valid, but it won’t be automatically renewed when the current subscription period expires.

If you want to test refund (not a regular cancellation), then it’s impossible as far as I know.