Not in the UK. Vehicle insurance in the US generally means anyone can drive/ride anything (through either your insurance, the owner's insurance, or I guess conceivably both). But in the UK, you are only insured to drive your own vehicles unless someone else makes you a named driver on their vehicle's policy (at an extra cost).If you have full coverage on your bike, wouldn't your motorcycle insurance cover the damage to the dealer's bike? It might vary by country and carrier, but I am covered for any bike I ride (not garaged at my address over 10 days) with the same coverage I have on my bike.
An interesting situation a friend in Las Vegas ran into: he had to sign a purchase agreement before the dealer would let him test ride a bike. There was a provision that if he didn't like it, the agreement was off. If he wrecked the bike, I assume he had to buy it.
The whole "I can ride/drive any bike/car and, with the owner's consent, be insured" was a pleasant surprise to me when I moved to the US!
The dealership in this case will have insurance on those bikes, but with a high excess. The guy who dropped it on the test ride will be paying the excess as per something he signed before taking it out.