MBP 2011 Fail GFX Passing Tests
I recently brought a MBP 2011 15" that was affected by the dedicated graphics issue to an Apple Authorized Service Provider (because that was more convenient than taking it to the next Apple Store). When they ran the test, they didn't find a problem either and then told me that they would not do the repair.

When I showed them the photos that I had taken when the graphic glitches occurred, they said that that was enough evidence and then they repaired the MBP.

Did you show the pictures/videos of the problem to Apple? If you did, I would either go to an Apple Authorized Service Provider and see if they agree to do the repair or go back to Apple and try escalate the problem. If you have photo/video proof of the defect, I would hope that it should not be too difficult to convince them to do the repair.