Replacing "apple hardware test" with "apple diagnostics"
So you didn't write this much in your original post but I am going out on a limb and gonna say that the AHT that boots doesn't work on your Retina MBP.

The only way to get the file back is reinstall the OS via internet recovery but if the /System/Library/CoreServices/.diagnostics folder is there then I would wager a small bet that internet recovery won't put Apple Diagnostics on the drive. 

You can test this by deleting the current folder on your MacBook Pro and then trying internet recovery. OR (I'd first recommend this) you could start your computer into Internet Recovery and reinstall the OS on an external drive. Once the install is done just copy over the aforementioned .diagnostics folder and then move it onto your MacBook Pro's internal drive.

All the best and much fun ahead! (Please give internet recovery 1-2 hours depending on your internet connection and ethernet is highly encouraged)