At what pressure would you carry out such a test? If a joint is blown by the test is the installer then responsible for it? As far as I'm concerned if a customer insists on changing to a sealed system they accept the existing installation can take it. If it can't, putting it right is at their expense. I would only pressure test and guarantee what I fit.
But it may not be the customer who insists on a sealed system boiler. He may say he he wants to get rid of the HW cylinder to free up the space. So the installer recommends a combi boiler. (How many combi boilers are there which are not sealed? I can't think of any.)
So the customer takes the advice of the installer and has a combi boiler installed. Then the problems arise - leaking valves (easily fixed), pressure loss which cannot be located. The customer will, quite rightly, say to the installer, "You didn't advise me of any potential problems so I'm not going to pay to put them right. That's down to you."