Google's John Mueller in a Google Webmaster Help thread stated that testimonials are not reviews and thus should not contain the rich snippet markup for reviews.
John said:
I'd just like to add that testimonials are not reviews, and that it generally doesn't make sense to use reviews like this for a company's website. My recommendation would be to remove that markup.
Reviews are more aimed at product reviews versus how much you like or dislike a company. So to include reviews markup on testimonials, is against Google's guidelines. As it seems.
Forum discussion at Google Webmaster Help.
