What is someone who makes a testimonial called?

In the context of advertising/marketing a testimonial is:

• Written recommendation from a celebrity or satisfied customer affirming the performance, quality, and/or value of a product or service. Testimonials are one of the most potent tools of marketing. (Business dictionary.com)

What is the term used to refer to someone ( generally a celebrity) who makes a testimonial?

Consider endorser. Oxford Dictionaries Online defines endorse as

Declare one’s public approval or support of: the report was endorsed by the college

When a pejorative term is desired:

sellout

Someone who does something that does not agree with that person's beliefs or values especially in order to make money