This is a little late, but this is what I did to exclude testimonial categories from Category widget.
Add this to your child theme's functions.php file:
function exclude_widget_categories($args){
$exclude = "8,10"; // The IDs of the excluding categories
$args["exclude"] = $exclude;
return $args;
}
add_filter("widget_categories_args","exclude_widget_categories");
This fix does not work on my site. Perhaps it is now outdated?
