How can I spin a dynamo wheel at road speed for bench testing? I can check my dynamo works by spinning the wheel on the workstand or just riding, but I'd like to do some more involved testing. I'd like to develop some custom electronics to use the extra power available at high speeds (for charging),  to drive an additional rear light that's always on even if the front light isn't, etc. The first step is to measure the output. I can make a start with a battery oscilloscope in my top tube bag, but that won't get me very far. 

Road speed is something like 300 rpm, and I can't achieve that by just spinning the wheel by hand, let alone work my oscilloscope at the same time, so I'd like a hands-free solution. Ideally this would work on the bike, as I'd like to try this with the existing lights as well.

I have tried running the chuck of a cordless drill against the front tyre, handheld. The speed ratio should be about right given the diameters of the chuck and wheel, but there's too much slippage from the hard curved plastic. Something cheap and easy to fit/remove would be ideal.
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)