Trouble adding arbitrary menu items to Walker_Nav_Menu

I am trying to manually add two items to my menu because I want them to be (linked) images, and I don't believe there's a way to do that without some custom code. Going off the instructions in This answer, since my case was a little different, I added the following code to my functions.php file:

class Custom_Walker_Nav_Menu extends Walker_Nav_Menu {
    function end_lvl( &$output, $depth = 0, $args = array() ) {
        $indent = str_repeat("\t", $depth);
        $output .= $indent . '<li>';
        $output .= 'Testing';
        $output .= "</li>\n";
        $output .= $indent . '<li>';
        $output .= 'Testing';
        $output .= "</li>\n";
        $output .= "$indent</ul>\n";
    }
}

I thought that would print out all of the real menu items, then append the two "Testing"s to the end before the </ul>. In my header.php file, I altered the wp_nav_menu call to:

<?php wp_nav_menu( array( 
    'theme_location' => 'primary', 
    'depth' => 3, 
    'walker' => new Custom_Walker_Nav_Menu() ) 
); ?>

I also tried removing () from 'walker' => new Custom_Walker_Nav_Menu() because I saw that suggested both ways in various threads here.

No matter what I do, though, the site never implements my "Testing" placeholders. I think the problem is that I'm placing something in the wrong place in my file system or naming something incorrectly, because what I'm seeing whenever I try this is just the normal nav menu output. I've double-checked, though, and I am editing the header.php and functions.php files inside my custom theme. Does anyone have an idea where I'm going wrong?