I worked at the GM Milford Proving Grounds for almost 40 years as a Test and Development engineer. I drove company cars as part of my job for evaluation, mileage accumulation and durability needs and also as a benefit. In the OP's picture, the key identifier is seen on the dash. It's a form of a decelerometer used by the test driver to put a controlled amount of braking force into the vehicle. This is done for durability and brake wear tests. It's heat, pad life, traction and ABS control stuff (stability control algorithm etc.). It is not uncommon to have GM owned competitive produced vehicles on test, too. (Toyotas, Hondas, Nissans, Fords and Chrysler, etc. This is done for performance evaluation baselines and comparisons.
The M plated cars are licensed for on=public highway test operations. The driver must fill out a log book entry to account for ever mile showing on the odometer. A non-test PEP car (Product Evaluation Program) has a civilian plate and a special sticker on the front windshield identifying it as a GM owned vehicle with a GM Fleet Operations. Blue is usually Engineering Staff. White is Divisional or supplier staff.
The purpose of the bras, covers, Tupperware, checkered decals is not necessarily to hide underneath new styling parts. Its to discourage the auto press from distorting the images that they want to publish as 'spy shots'. They stretch, patch, shrink, narrow, lower and modify their lunch-time photos to present their incredible secret discoveries as exclusive magazine lures.
Good example is the recent Lincoln ad comparing it to a Cadillac. The Caddy images have more Adobe botox in them than a 12 foot snake in order to make the car look more unattractive.
If this particular car was a CTS-V, then its possible that it was run on the 'Ring. But heck, you can pay $5 and run a bicycle and get a sticker. Sorta like going to an amusement park. A Corvette on the 'ring gets a lot of respect from the BMW's, Audis and Ferraris. We often teased a motorcycle on that road and left it in the weeds. The bikers always were amazed that it was just a production car and not a specially prepared one of a kind. Even a CTS-V rental car can smoke just about any car over there. Yes, it takes a driver with some nads...
