When your endpoint is inactive I think you have to send a post request to auth.net webhooks endpoint to test and that the test button won't deliver a payload.
There is a section on the documentation "testing a webhook" that explains how this test works. You have to base64 encode your login credentials before making the request. It is all explained in the docs.
If you're consistently getting a payload from actual sandbox/production transactions then IMO you don't need to do any further testing to validate that your endpoint works. Doesn't really matter what it does in inactive mode.
