Who says stunt?
As you can read in the post higher up on this page, KLM's test flight was conducted in cooperation with IVW, the Dutch CAA,
The flight is mentioned on the official web page of IVW as being conducted on request of the European Union. More test flights in other countries are to be conducted.
As far as test equipment is concerned, of course no special equipment is added to the outside of the machine. How about borescoping the engines after the flight to look for the slightest traces of ash EFFECTS. You may have found the pictures of the Finnish Air Force F-18 engines earlier on in this forum. The civilian test flight are there to establish whether any signs of ash can be found after a flight in a thorough technical inspection, not just from the observations of mr. Hartman or mr. De Haan.
The risk is not in this one short flight, but flights such as this one may indicate whether there will be any long term risk in starting up full scale operations.
If you are concerned about money, even if both engines would now need a massive overhaul, that is money well spent to prove that losing 5 to 10 million per day per company by not-flying is justified.