When I used to work at the VW dealer in France this was not uncommon practice though we always informed the client if we were to do this and if the client refused of course it was not done. Usually though they would not mind if we threw in a set of floor mats and a full tank. Maybe the dealer you tested the car with asked for the client's permission before offering you to take it out. Just a thought...
Nevertheless I notice that the test drive policies are very different from country to country. Over here more often than not they slap a set of dealer plates on the car you wish to try, throw you the keys and let you go off for a drive on your own... and no it's not only on old bangers, I've seen this exact practice happening at a Ford dealer while I was just looking around and the salesman "sent off" a couple in a brand new Focus ST...