How can I spin a dynamo wheel at road speed for bench testing?
Sounds like you have the same requirements as the Rubbee, which is a small wheel pressed against the tread of the tyre.

Have you got an old inner tube (perhaps a 2" MTB sized one) where you can cut a slice off, and mount that over your drill's chuck?

If the diameter of the chuck isn't quite large enough, a wooden disk with a larger radius could be mounted in the chuck with a bolt on the disk's center.

Don't forget that some drills have gearboxes with multiple stages.  2 speed is common.

A last resort might be to disregard any battery powered drill and check the effective RPMs from a mains powered drill.  

I've successfully mounted a spoke magnet in my pillar drill and used it to test a bike computer sensor.  (It topped out reading 150 km/h)