Simply put its not mentioned in the bike testers manual so not testable, does not matter if its not there or the bulb is blown i cant test it anyway………
It is a requirement for cars as its mentioned in the car mot manual & most bike testers will stick an advisory on the MOT just to cover his own ass as you can get nicked for it being broken or missing in use on the road under the road traffic act 1988 & we dont want you coming in shouting if you get nicked
This one in particular surprised me as ive been failed on a bike before for a broken number plate light !