The pain depends of the material the seat is made off and from wich hight you let it fall on the testicles.
The plastic t0ilet seat don't hurt so much but the heavy wooden ones are real painful.
But both won't destroy your testicles and the pain will fade rather fast.
So, it is best you start with letting the seat fall from a limited hight and build up slowly. Balls are pretty strong and they will warn you with unbearable pain before you do permanent damage to them.
Examples on my ThisVid page.