Hi Jürgen. To test an Xcode 8 / iOS 10 build on the iPhone X simulator you need to follow these steps:

• build the app on Xcode 8 (obviously!)
• find the .app file that it produced. The simplest way to do that is to expand the ‘Products’ folder in the Xcode project navigator, right click and choose ‘Show in Finder’. Copy the .app file somewhere safe (e.g. the Desktop)
• quit Xcode 8 and, if it is running, the Xcode 8 simulator
• launch Xcode 9 and run any app to open the iPhone X simulator (you can probably manually launch the simulator, but it’s quickest to just run an app and have Xcode do it for you)
• drag and drop the .app file from your desktop to the simulator to install it. You might need to be on the home screen, I can’t remember
• then you can just run the app by tapping the icon on the home screen

You won’t be able to debug the app, but you will be able to run apps built with Xcode 8 on the Xcode 9 / iOS 11 simulators this way.