In the emissions test, the car would be held between 2500rpm and 3000rpm for 19 secs. That's from an already 'warm' start. If it passes this fast idle test, another 60 seconds at these revs to stabilise the cat before it is returned to idle. And that's only if it passes the test criteria. Should it fail in that first 19secs (which becomes 30secs if failing)of time, there is a vehicle specific procedure. This is a repeat of the previous attempt, sometimes some of the test limits, are slightly altered (this doesn't happen that often). If it doesn't pass this time the machine will prompt the revs to be held at the above level for 3 mins. And then repeat. We would be obliged to follow this procedure to give your car a fair chance to pass.
The thing here is that the car is probably marginal on emissions. In practice (if using an engine speed measurement) I would move the revs higher in an attempt to 'light' the cat. This stops the countdown and when the revs return to the correct range the countdown restarts. On 'difficult' cars I would do this two or three times before admitting defeat. I have no way of knowing if a car will boil up in this time but it's something I would look out for. Saying that, it's happened umpteen times. It's not my fault, I didn't set out to overheat a customer's car. This may have been what happened to yours.
Neil, what I'm saying is I feel your pain but spare a thought for the garage who is only trying to test your car and as a consequence of some fault the car overheats. They didn't set out to overheat your car or fail to test it! No amount of revving a stationery car will overheat it if the systems on the car all function as intended.
One last thing - did they charge you for the test?