I was told that in a Roman court of law, a soldier would hold the balls of a witness while he was speaking. If the witness was found to be lying, then the soldier would exact punishment. Hence testifying. I'm not sure how true this is. Could an historian confirm or reject this?
I think this is just a modern day myth because of the relation between the word testis and testicle. I am not aware of any primary evidence which describes such an action.
From the online Etymology Dictionary
from Latin testis "testicle," usually regarded as a special application of testis "witness" (see testament), presumably because it "bears witness to male virility" [Barnhart]. Stories that trace the use of the Latin word to some supposed swearing-in ceremony are modern and groundless.
Basically, your testicles were the 'witnesses' to you being a man.