Do not link to the posters of the testimonial, unless it’s relevant for you

If you have testimonials, they may be from people from large companies. Some websites add a link to those testimonials that leads to the sites of said companies. Don’t do that. All you’ll accomplish with that is that your visitors will leave your site.

A logo is more enough to bestow trust upon your readers, and if they really want to know more about that company they can look them up themselves. 

Your site is about you. Make sure that the testimonials always remain a marketing tool for you, and not for another company.