They’ll test it for the weight, however it’s important to understand the testing cycle. Were the tests only setup on flat ground? Did they adequately test braking? Did they ride the brakes for a while?

Well, it seems that you are not familiar with European road safety testing and certification. My BIL was working in that area, and his executive summary of that was: if it passes European tests, you can sign the American test papers, too, as it would pass them with flying colors.

Will they test it? Absolutely! Every year when I extend registration on vehicle everything gets tested on a car. Vehicle must pass technical exam which lasts some 40minutes. Breaks are tested the longest among other things. They measure not only breaking force but consistency, elongation of disks if you have them, everything.

Of course there are still people out there who don't maintain their vehicle properly after that exam, but at least once a year it has to be in tip top shape. Also if you have a towing hook by new regulation it has to be tested as well and removable. It's actually illegal to drive with it if you are not towing anything, since it effectively negates crumple zones.

People are constantly complaining about how strict these rules here are, but I don't. They save lives.

As far as I know, the maximum unbraked trailer mass is 750kg in Europe, although for some cars it can be lower.

With a braked trailer the limit is up to the manufacturer (and your driver's license).