How do I include testimonies in my category or archive view

At the beginning of your theme’s functions.php file, place code similar to the following for the configuration you need.

add_filter( 'pre_get_posts', 'pre_get_posts_allow_testimonials' );
function pre_get_posts_allow_testimonials( $query ) {
    if ( $query->is_admin ) {
        return $query;
    } elseif ( ( $query->is_main_query() || is_feed() )
        && ! is_page()
        && ( ( ! empty( $query->query_vars['post_type'] ) && 'post' == $query->query_vars['post_type'] )
            || is_archive() )
    ) {
        $query->set( 'post_type', array( 'post', 'testimonials-widget' ) );
    }

    return $query;
}