I've been working through the exercises in the excellent iOS Unit Testing by Example book by Jon Reid, which I highly recommend. However, the book is in beta at the moment and there are some curveballs thrown by iOS 13 that aren't handled in the text yet. Specifically, when I hit the section about using a testing AppDelegate class I thought "This is very good. But what about the SceneDelegate?"
In Chapter 4 the recommendation is to remove the @UIApplicationMain decoration and make a manual top-level call to UIApplicationMain. To wit:
import UIKit