If you want to include a random comment from one of your posts you can use the following code in your theme:

<?php $post_id = 33;
// Put the 'testimonials' id here
$comments = get_comments("post_id=$post_id&status=approve");
if ($comments) { $ndx = mt_rand(1,sizeof($comments)) - 1;
$comment = $comments[$ndx]; }?>
<p>
<?php echo $comment->comment_content; ?>
</p>
<p>
<?php echo $comment->comment_author; ?>
</p>

Replace “33” with the post ID you want to get your comments from. Such technique is often used to get comments from post that asks for feedback for example.