I've started writing this two or three times. It's so much bigger than I think it will be every time. This would need to either be a big time investment, or a community effort to supply all the tests one can run. And doing a speed test to determine what the bottleneck is in network speed requires a lot of third party bandwidth -- heck, even a million people pinging your server every day would be something to consider if your server is a potato (mine is).

That said, I'd love to see (and/or contribute to) such a tool too!