Yes CarMax does allow people to test drive cars they sell and you should always test drive a vehicle to see how it drives, runs and operates and to determine if you like the car or not before buying.
CarMax does allow Solo test drives and if a car dealership doesn't allow you to test drive their vehicles then something is wrong and you should avoid that dealership and look for a different one.
All car lots and car dealerships who are in the business of selling either used or new cars should allow test drives and if they don't then they're not likely legit.