Add suffix to price html does not work

I don’t think the filter will work in your template, since you’re using wc_price( wc_get_price_including_tax( $product ) ) .. a possible workaround:

function pr_price_suffix( $price, $product ) {
 return wc_price( wc_get_price_including_tax( $product ) ). ' test';
}
add_filter( 'woocommerce_get_price_html', 'pr_price_suffix', 10, 2  );

and in your single-product.php template:

 <?php echo $product->get_price_html(); ?>