If it’s not connected, then they can’t fail it, but you really need to show that it is disconnected. What car is it?
From the testers manual –
Additional stop lamps
Any additional stop lamps fitted and connected must be tested. Where extra lamps are fitted and there is doubt as to whether they are connected, the benefit of this doubt should be given to the presenter
High level stop lamps
These are lamps usually fitted in the rear window or boot spoiler of a vehicle and may consist of a number of light sources .
At least 50% of the light sources must illuminate
However, a high level brake light is classed as an additional light with regards to the MOT. Plod and insurance can still do you for it not working though.