The testimonials for that module look hardcoded and configured from the modules configuration. So I don't think you can really push dynamic content to it like that. You could create a testimonial field that they can edit from their profile then you could have a userlist with no search functionality to display those testimonials (be sure to filter it to only users with a testimonial field value). Next you can load that userlist into a module using Joomlas wrapper module.

Is there a testimonial thing here on CB ? Can I see on this site a running example ?

We have some testimonials on the CB page below, but I'm pretty sure those are manually entered.

www.joomlapolis.com/community-builder

We primarily just use Joomlas extension directory for testimonials though.

And is there a tutorial that I can follow to set this up?

No not this case specifically, but there's tutorials covering basic userlist usage, which can be found in your CB documentation that you can download from the Advanced downloads section. For your case create a textarea field for the testimonial. Then create a CB userlist that filters on that field to check that it's not empty. Next add columns and fields to the columns however you like.