The phrase "further testimony" refers to additional statements or evidence provided in a legal or formal setting. It means more information or proof is being provided to support or clarify a situation or argument. Full definition
And a girl, on hand to be rescued, fought over and filmed from behind at a low angle as further testimony to the aesthetic power of 3 - D. (nytimes.com)
The prosecution counsel, however, sought an adjournment to allow him present more witnesses to give further testimonies. (nigeriapoliticsonline.com)
Other stations provide further testimony into the rebel's life and background, everything but what designer he's wearing. (consequenceofsound.net)