How can I set up a simple speed test on my site?

Go to http://www.speedguide.net/speedtest/add_mirror.php and download http://www.speedguide.net/speedtest/data/sg.zip from there. Unpack it. You’ll find some files with test data in it (random binary data in exact sizes). Choose some of them and upload them to your site. Then use wget with the URLs that correspond to the speedtest images, and it will give you the average download speed for this data.