The New Testament shows the promise fulfilled. The promise that God gives us in Scripture is the promise of forgiveness and eternal life, purchased by the blood of Christ. It tells us that Christ is God and that he became man. It tells us Christ lived a perfect life on our behalf. It tells us Christ gave his life as a payment for our sins.
Testimony is what is given in a trial.
Scripture is how God saves us. He could create faith while we are sleeping, eating, or playing a round of golf. Through the gospel message, the Holy Spirit creates and strengthens saving faith in Jesus Christ. Scripture is where God speaks to us. It is where God tells us about our Savior.