You only gain experience if they race, running test laps has no impact. Its per race and like Blunion said, I think its very small per race.
also, remember that experience counts to the drivers star rating... so the more experienced they are they higher the star rating and the slower they train....
I find it really doesnt count for much. I have 2 drivers, 20 across the board one with exp of 5 the other 13 and they finished within 6 point of each other, 1st and 2nd in the league. The guy with 5 exp regularly was just faster than the "more experienced" driver.
The more experience they have the better they are. less wee mistakes, once they get to 20 they can push a button without looking at the steering wheel, it becomes second nature to them.