The test kit only have one port on the sidecar but the dev kit has too. The port is still there but covered up under plastic.
The number of USB ports on your sidecar (or the type of sidecar you've got) is totally irrelevant to whether or not you own a devkit vs testkit, the old kit sidecars used to have a PIX USB port (which was not functional on testkit as it wasn't supported at the kernel level, although you could still plug it in), which has later been deprecated in devkits and testkit alike because the PIX functionality has been build into the kernel (to be specific it is now part of kdnet).
Because developers no longer needed to use the aforementioned PIX port, but Microsoft had yet to renew their sidecar parts, they eventually shipped the old sidecars with PIX hardware but a custom plastic shell that covered the port, eventually they switched to a new sidecar that did not include the PIX hardware at all, sidecars ever since the Zephyr era were used for DVD emulation exclusively, I believe the PIX hardware was removed from the sidecar by the time Jasper/XNA XDKs shipped.
This means you can have testkits and devkits tha have no PIX port (or at least none that's accessible out of the box) as well as testkits and devkits (usually xenon) which do have the PIX port available for use, obviously the is no point in having the PIX functionality on a sidecar unless you are using very old recoveries and XDK software, it's nice to have as a collectible I suppose, but adds nothing functionality wise since it is entirely defunct and deprecated.