I would think that the Skimmer would take the DT test before it moved. I would say this because it has started its move in DT and thus would need to take the test BEFORE it moved, if it fails it it does not move as teh Vehicle is now Imobilized. As far as word sniiping and trying to get a quick kill, the fact that the owning player said he WANTS to go flat out doesnt mean that he/she was able to. I would think that one would count as going Flat out ONLY if it moved the required FLAT OUT distance. You can claim any bonus for SAYING you are going flat out but only go say 6" so why would you get penalized on the inverse?
So if the owning player SAID they WANT TO go flat out but fail the DT test right off the bat the skimmer would sit were it is, now being immobilized, since it hasnt really moved at all.