You already have the right “action”, which is woocommerce_after_cart_contents.
But when I tried the markup you used:
function quality_certificates(){
echo '<tr>TEST</tr>';
}
this was the visual output: (I actually initially tested with the Twenty Seventeen theme; sorry about that. But this one is tested with Storefront)
Then I started thinking that the problem might be the markup, so I changed it to:
<tr><td colspan="6">TEST</td></tr>
and voila! I got the expected visual output:
So, use the proper markup/HTML. =)
PS: Those are actual screenshots. 😉