Aerodynamic drag and rolling resistance is factored into the testing by the manufacturer doing a "coast down test". Vehicle is run along a test track at a set speed and allow to coast to a halt. The distance travelled etc is used to determine drag etc and is used as described below. Manufacturer has the opportunity to massage this important data as described by the BBC article.

nptel.iitm.ac.in/courses/112104033/pdf_lecture/lecf

Driving Cycles for Light Duty Vehicles

"The emission test driving cycles are composed of a cold start period, idling, moderate acceleration and deceleration, and cruise modes. The test cycle is given in terms of vehicle speed versus time. The light and medium duty vehicles are driven through the prescribed driving cycle on a chassis roller dynamometer. During operation the engine is required to develop road horse power that depends on the vehicle speed for a given vehicle. The road horsepower requirement versus speed data as provided by the vehicle manufacturer or determined by vehicle coast down test is stored into chassis dynamometer controller to simulate the real life road operation of the vehicle. The vehicle weight i.e. inertia needed during transient modes of the driving cycle is simulated by mechanically changing the rotating masses or electronically changing the inertia on the roller dynamometer."