Currently it’s indeed very hard to find a test and it’s a bummer.
However I had a chance to discuss this with the head of the certification department at unite Europe and unity is pursuing a partnership with Pearson (16.000 examination centres all around the world). And soon ™ all those centres will be hosting unity cert examinations all year round which is awesome news.
Amongst other news, he told me that the Unity Professional Programmer cert will be rolled out in January 2018