Well, of course, there is no such thing as an e-bike license. You would be taking a moped test (at least). You would then be able to ride one of these US-spec machines but it would be registered here as a moped and would need a numberplate. One qualification in the type-approval process would be establishing a design speed of less than 30mph under power.I can't see anyone getting that license riding an ebike with a 20mph limit. The way you would have to ride it safely would be completely different to what the official at the test center (would would probably conduct the test on a large motorcycle) would expect.
You would have to do the test on a moped or 125, not exactly ideal just to get an ebike license.
You could ride that on L-plates, just as you would any other moped. If you had a non-recent car license, you would not even need the L-plates, but in all cases you WOULD need third party insurance.
What the reaction of the driving test examiner would be if you turned up on a registered (i.e. with numberplate) electric bike, I don't know. Technically he/she would probably need to accept it.
Rog