This app provides the server needed for testing with the free Network Speed Tester Client app for iOS and tvOS.
Ideally, the device running the server app is close to your WiFi router. You then use the client app on any iOS device and move around your house to measure WiFi performance. Please note that the built-in network port on the Apple TV is limited to 100 MBit/s.
There is also a combined client/server app available for the macOS platform called Network Speed Tester.