The airbag warning light indicates that the airbag might not deploy if the vehicle is involved in an accident.
MOT test rule 7.1.5 states that "an airbag obviously inoperative" is classed as a 'major' fault and therefore an MOT failure.
Further rule 7.1.6 states that a supplementary restraint system (SIL) malfunction indicator light (MIL) being on is also a 'major' fault and thus, once again, a reason for the vehicle to fail the test.
Irrespective of that though, why would anyone want to drive a vehicle that's got an inoperative safety feature. Get it fixed!
[Note: If the car was involved in a minor bump, which might have been nothing more than bumping it over a kerb to park on a verge, it could have triggered the first stage of the airbag's deployment system but without actually inflating it. That's all that's needed to get the warning light to come on. If that's the case, resetting it should be a simple job].