If it is "insecure" it will fail, or if it is "obscured, damaged or deteriorated so that the view to the rear is seriously impaired" then it will fail too.
Depends on how bad it's damaged really and on the testers' interpretation of "seriously impaired".
You'll get an advisory on that one, so long as there are no shards of glass projecting.
A motor factor can probably sort you out with a stick on replacement though.
John