class Event implements EventSubscriberInterface
{
    /**
     * @return array
     */
    public static function getSubscribedEvents()
    {
        return [
            'Block/category_nav_pc.twig' => 'onRenderBlockCategoryNavPc',
        ];
    }

    /**
     * カテゴリナビ(PC)にカテゴリアイコンを表示する.
     *
     * @param TemplateEvent $event
     */
    public function onRenderBlockCategoryNavPc(TemplateEvent $event)
    {
echo "OK1";    // ★
        $event->addSnippet('@tbsTest/default/Block/category_nav_pc.twig');
echo "OK3";    // ★
    }
}

OK2    {# ★ #}
{% set product_list = url('product_list') %}
{% for Category in Categories %}
    <script>
        $(function() {
            $('a[href="{{ product_list }}?category_id={{ Category.id }}"]').html(test{{ Category.name }});
        });
    </script>
{% endfor %}