I do not think they are tested, other than by the designer nor are they embroidery machine brand specific.  I do not download very many free designs, but on occasion when someone else has had a problem I have downloaded the design in question.  I will give this one a try and let you know my experience.

ETA - stitched out as well as any free design.    I used a cut away stabilizer, 505 spray adhesive because the design is dense.  Sulky brand is easier to find.  Cut the stabilzer to a little larger than hoop size, then lightly spray the stabilizer (you can hold it down into a large wastebasket to eliminate over spray).  Hand press he fabric to the stabilizer and then hoop.  Work on a firm surface and not the soft ironing board.

I also used a size 14 titanium coated machine embroidery needle.  I use the coated needles for everything as well as when using sticky-back or spray adhesive.  This type of needle does not heat up like an ordinary machine embroidery or sewing needle, which can cause gumming and thread breakage.

Refer to your user's manual and clean the bobbin case and surrounding area.  Make sure the bobbin is evenly wound and the thread is feeding snugly through the tension guide on the side of the bobbin case.

Test another design to see if the problem of "bird's nesting" and thread breakage continues.  If it does, the machine will need to be serviced.  Take a sample of the problem stitching with you (I put it under the presser foot) so the tech can see what the problem is.

Hope this will help.