Decades ago, microphone test heads were standard products that any decent-size studio would have on hand for use by their staff maintenance engineers. But the technology and (even more) the economics of the studio business have changed since then. The only manufacturer I know of that still makes test heads (a/k/a "measurement adapters") available to the general public is Schoeps; the one for the Colette series is called the MEC.
Neumann used to sell test heads for their various models up through the fet 80 series (MA 87 for the U 87, MA 84 for the KM 83/84/85, etc.), but hasn't done so since they started building their microphones with custom integrated circuits in the 1980s. I've never seen test heads on an AKG price list, but I'm sure they were available behind the scenes "back in the day." Perhaps they and other manufacturers can be approached with private requests; they all have to make them for their QA and repair departments to use, and many manufacturers have overseas repair stations, so it's a question they all have to deal with one way or another. They might even be persuaded to lend you one. Smaller, less mass-production-y companies can sometimes be surprisingly flexible.
Alternatively, with some care test heads can be improvised. Electrically they are just series capacitors with the same capacitance as the capsules they're meant to replace. The mechanical construction (w/r/t shielding and stray capacitance) is important, especially given the ultra-high impedance of a condenser microphone's input stage, but it doesn't need to be the ultimate concern for listening tests where the noise floor or the lowest possible distortion aren't the main issues.
--best regards