。
She testified that she had seen the man.
Literal
She [topic-は] that man [object-を] saw [quotative-と] testified.
証言する is the formal verb for giving testimony — typically courtroom or sworn legal statements. The quotative と embeds the witnessed event ('that she saw that man') inside the act of testifying, the standard frame for reporting any direct statement in Japanese: [content]+と+[verb of saying]. Other reporting verbs in this family (述べる, 主張する, 申し立てる) each carry their own register, but 証言 is specifically reserved for formal, often legal, declarations of what one observed or knows.