You're only loading the parser, but you aren't doing anything with it. You need a line like
$my_html = Markdown::defaultTransform($my_text);
...to convert markdown to HTML. Like this:
<?php require_once ('subfolder/MarkdownExtra.inc.php'); require_once ('subfolder/includetest.php'); ?> <html> <body> <?php Markdown::defaultTransform("[Test](www.google.com)"); ?> </body> </html>
I haven't used this before. I'm just going by the docs that I can find.