Testing GUI applications is difficult in any context. I don't know of anyone doing this with Java in CodeRunner. It might in principle be possible to hook into a headless JavaFX testing framework, but unless someone else jumps in to help, you're on your own here.
Foro
Question Authors' Forum
Test JavaFX ?
Este foro tiene un número limitado de mensajes para enviar en un cierto período de tiempo. El ajuste normalmente se hace en 5 mensaje(s) en 1 día