The "No Preview Available" error sounds like it is coming from Google, so it seems like the iFrame is loading, but it's just that Google can't access or render the test.pptx document.
There may be an error in your web browser's console if you enable the browser's developer tools and then view the page with the iFrame on.
Does the URL you are using for the iFrame's src work when viewed directly in a browser outside the iFrame?
Good luck!