As "FriendofTruth" stated, "testimony" is an accounting of the TRUTH. What that truth then reveals could either convict or exonerate, depending of the subject and case at hand.
Therefore a "testimony" is NOT ALWAYS used to reveal a truth AGAINST someone or something (although it often is in a Court of Law). It is intended however, to be NEUTRAL and UNBIASED just for what it is, "a statement of the facts".
In law and in religion, testimony is a solemn attestation as to the truth of a matter.
The origin is "the Indo-European roots *tre- meaning 'three' and *sta- meaning 'stand'. A witness was 'a third person standing by'. From that came the verb testificare 'to bear witness', which evolved into Middle English testify in the fourteenth century."
In the law, testimony is a form of evidence that is obtained from a witness who makes a solemn statement or declaration of fact. Testimony may be oral or written, and it is usually made by oath or affirmation under penalty of perjury. Unless a witness is testifying as an "expert witness", testimony in the form of opinions or inferences is generally limited to those opinions or inferences that are rationally based on the perceptions of the witness and are helpful to a clear understanding of the witness' testimony.
So in answer to your original quesion--I would say that in the BIBLE, a "Testimony" would be as in an "expert witness" on the behalf of God, in the support of, not against.
Jesus saith unto him, "I am the way, THE TRUTH, and the life: no man cometh unto the Father, but by me." (John 14:6 KJV)