Dear NAME_RECEIVER,
Recently, I have been working on the product named PRODUCT_NAME. After a few testing, several problems about this product have been spotted so I continued to make some amendments on it. I think it is well fixed now but I still need someone's help to verify it. You are the most suitable person I can think of for this.
[Insert the problems and functions of the product that you would like your co-worker to test on].
Thank you very much for your help. You can feel free to give your opinions on this product for improvements!
Yours sincerely,
........................................