Drupal 8 - How to add classes to link (render array) without using preprocess....

menu-local-task.html.twig

{#
/**
 * @file
 * Default theme implementation for a local task link.
 *
 * Available variables:
 * - attributes: HTML attributes for the wrapper element.
 * - is_active: Whether the task item is an active tab.
 * - link: A rendered link element.
 *
 * Note: This template renders the content for each task item in
 * menu-local-tasks.html.twig.
 *
 * @see template_preprocess_menu_local_task()
 *
 * @ingroup themeable
 */
#}
{#<li{{ attributes }}>#}
{% set link = link|merge({'#attributes':[{'class':link['#attributes']['class']|merge(['test'])}]}) %}
{{ link }}
{#</li>#}

Had similar issue, but im displaying terms rendered as labels, i need to add some of my own classes.
In my code, this: link['#attributes']['class'] is null so i came up with this. Also, in the top of the twig i do some logic to understand which class associate to the term.

<li class="list-inline-item">
    {% set term = term|merge({'#attributes':{'class':btn_classes}}) %}
    {{ term }}
</li>