You should test on the same surface like concrete or a paved road.
Same weight distribution
on both sides of trailer.
(Both tires have the same friction on them.)
Then test about 5 times going about 20 mph. If the brake for one wheel is not working at all, the trailer will veer to the right or to the left.
If the trailer goes straight when just the trailer brakes are applied, might just need to adjust the brakes. Many trailer brakes are not self adjusting.
Adjusting trailer brakes...
If the trailer does veer to the right or left, check the wiring to the non working trailer brake with a multimeter. Be sure you have a good separate
ground via the trailer electrical connector and are not using the hitch as a ground source.
Also the brake electromagnetic coil could be toast. You can check that with an ohm meter. Here is a manufacturer of trailer brakes. They should have instructions on their web site somewhere...