How to run latest tvOS SDK on macOS 10.13?
This is obviously not supported and I don't know what wackiness may result from it, (some weird stuff happened when I did it myself) but this is where you find and put the SDK's for specific platforms in an Xcode version.

On the new Xcode (the one with the SDK that supports your device/OS/etc.) you can find the SDK bundle itself in 

Xcode.app/Contents/Developer/Platforms/AppleTVOS.platform/Developer/SDKs 


Then extract (copy) the tvOS version you want to work with, as well as the simulators if you want to, and put them in the same path in the version Xcode that you currently run. Restart to see if they show up as targets.

As I mentioned, this isn't supported at all, and you might face more than trivial issues, but it's a starting point.