There are 2 thing you need to do here. The first this would be to create the ACF taxonomy field and set it to load and set terms.
The second thing has to do with showing the testimonials in each category. This is actually a core WP thing. You just need to create a template for the taxonomy. This works best when you create a custom taxonomy so that your testimonials do not get mixed into the standard post categories and visa-versa. https://codex.wordpress.org/Function_Reference/register_taxonomy. Then you create a template file named “taxonomy-YOUR-TAX-SLUG-HERE.php”
