get_the_content & include_once

Hi
I’m building my first plugin and I would like to know whether this is the correct way to set get_the_content() with including external php files. Almost is fine, but sometimes the content of test.php is appeared on the top of the page, outside the theme.

function Test($myContent) {
   $content = get_the_content();
   $incl = get_the_content(include_once("test.php"));
   $content = apply_filters('the_content', $content . $incl);
}
add_action('wp', 'myContent');

Content of test.php:

<?php
echo "Test";
?>

Thank you for the Feedback,
Lomo