Why use testimony?
Testimony is not the gospel — and it is the “gospel” that is the power of God unto salvation. Here are some reasons to make use of testimony:
Your growing relationship with your friend may not yet be at the point where the friend is willing to talk with you about the reasons for your faith. During that time, you can tell about times that God helped you. These are non-threatening, because you are not asking your friend to respond. They are one additional thing you can do during the time before you have an opening to share the gospel.
Sometimes a testimony can lead into talking about the gospel. For example, if you talk about how God saw your through financial difficulties, the listener might say “God would never do that for me — I’m not close to him, like you are.” This gives you a chance to say that God did not help you because you deserved it. From this you can go into the reality of sin in everyone’s life, including your own. That lays the groundwork for you to explain how Jesus Christ took care of the problem of sin. In explaining that, you have shared the gospel.
Public testimonies get attention, and can become a framework within which to share the gospel. A public testimony may or may not include the gospel — it is up to the speaker to add it on purpose.
When you share a “faith-story,” your listener might not agree that God intervened. Your listener might say “it was only a coincidence” or “it was only luck.” You cannot prove that it was God. You can only demonstrate that you are a person who believes in God. For the other person to come to believe in God, it is necessary to hear the gospel message.