The popularity of diet fads is a 'testimony to' the fact that people want a quick fix for their health and weight problems.
It is 'testimony to' her courage and persistence that she worked for so long in the face of such adversity.
There were contradictions in her 'testimony'.
(..) You think he'd just fall to his knees and cry, "Yes, I did it. I confess. By the way, add a life term to my sentence." With Tommy's testimony, I can get a new trial. That's assuming Blatch is still there. Chances are, he'd be released by now. They'd have his last known address. (..)1994 The Shawshank Redemption