WP PHP - Foreach adding different classname - how does it work?

Add the index to your foreach loop:

foreach ($postslist as $index => $post) :

and use that to append to the class name:

<div class="testrun<?= $index ?>">

As long as the array is an indexed array, you'll get testrun0, testrun1, testrun2 etc.

If you want to start with 1 instead of 0, just change it to:

<div class="testrun<?= $index + 1 ?>">

You can try something like this.

<?php

$i = 1;
foreach ($postslist as $post) :  setup_postdata($post); ?> 
    <div class="testrun <?php echo "item".$i; ?>">
        <img src="<?php the_post_thumbnail_url('blog-small');?>" alt="<?php the_title();?>" style="margin-right: 20px">
        <br />
        <!-- <?php the_date(); ?>
        <br />
        <?php the_title(); ?>
        <br />
        <?php the_excerpt(); ?>
        <br /> -->
    </div>
<?php $i++; endforeach; ?>