It depends ... Ben Collins seems to have the job for most of the drives, but other drivers may be used depending on the type of car and what the manufacturer wants. The iconic white race suit can be fitted to all sorts of test drivers, when a car maker wishes it (and considering the influence of Top Gear and its lap time board I'd guess they would do anything to present their car best and highest on the board).
And for the comedy roles he may be played by a simple actor, and certain actions like that snowmobile jump are reported to be done by experts anyway.