Pre-season testing would be a completely useless feature — there aren't months long off-season breaks between leagues here, most leagues here are racing everyday or three times a week. Nobody would be able to realistically utilize it quick enough.
Althought I do agree with Blunion under the current game setup.
However, with Jacks talk of a complete overhaul of the design process, and much less formulated setup, being able to take cars in different directions when designing them etc, I wonder would there be able place for even 1 "test day" if the league owner wants? Give the league owner the control. You might then give the the teams in that league a chance to change the direction they have gone with design etc?
Under the current game there is no point as there is no changing how you develope your car, the aim is to go for 100% as quickly as possible.
I expect in the next game there will be little point too due to time, but, depending on how the new "points system" works for design and how cars are not just going to be the average of of your % dev, but you will actually take your car in an actual direction (fast low aero, good aero less straighline etc). Then maybe, just maybe, there may be a place for this.