There is plenty of room in DCA for a testtrack style attraction. Timon lot is quite large and could hold several attractions including a full fledge testrack. The track could also be modified to fit within DCA also.
Notice how the concept art shows walkays and bridges over parts of the track.
What makes Testrack at Epcot look big is its
2) wide open area it uses for the track which was ok to do because there was plenty of land to build on.
A testrack attraction at DCA would use every possible corner and open space around the track to create walkways, shops, restroom facilities.
Basically incorporating several elements within the same land by using diferent elavations thruout the attraction including a show room for a darkride section.