The Simpsons: Night of the Living Treehouse of Horror (UE) (GBC)
FX Test And Music Test Accessible
Note: Press Down while highlighting Difficulty to show the two new options.
I believe this was disabled, I was not able to get it to function.
Although it could be unlocked from a password, this text is in the ROM:
“Well Done…….You have earned a secret password…Guard it carefully”