Testimonies are often used on advertising media for a company, mainly on websites, where positive comments and praise is posted on the site supposedly from previous customers.
The problem with this is that from a pessimistic point of view there is no evidence that the testimonies are actually real and a lot of people would automatically presume that even if the accounts are real only the positive ones will be published onto the website. If you are fully confident in your company a more openly respected form of promotion would be a link to an independent review website that shows both positive and negative feedback to give customers a balanced view.
This way any negative reviews can be responded to correctly and can make potential new customers confident in doing business with your company in the future.
If you have customers who are happy to promote your work you would be better to have them do it through social media linking to your website so that it’s evident that the review came from a real customer.