Custom Post Type testimonials can have categories settled.
Categories can be added and settled on each testimonial when editing it:
And in separate tabs under Testimonials:
You can implement such actions with Categories: add/edit name, slug, description and manage the hierarchy level. For instance, "post" category refers to parent "blog" category. Various notional connections can be emphasized in a such way:
Testimonial categories can be found in settings of Post Types shortcode: