From wordorigins.org:
"Testify
There is bit of folklore floating about that the word testify (with its associated forms like testimony and testament) derives from testicle. The legend has it that ancient Romans would hold their testes while swearing oaths. This is not true. The word does not derive from testicle, nor did Romans swear oaths on their private parts.
Testify, et. al., derives from the Latin testis meaning witness. Testicle also derives from testis. The testes are witnesses to a man's virility."
The point? If something sounds too ridiculous to be true, it probably is.