I have no idea what this thing is connected to and am just thinking along theoretical lines. the classic situation is back emf that has a negative and positive spike, on the positive side it needs limiting, on the negative side it needs clamping to ground.
Right -- even in the theoretical motor case, the negative spikes get dumped to ground, and the positive spikes get dumped to VCC. Either way, VCC is becoming
more positive with respect to ground, not decreasing let alone going negative.
Well, I'm not sure. Can i use every diode listed?
That link has had parameters preselected so it's only showing unidirectional diodes with a breakdown voltage in the 6-7V range, just like the one you specified in the first place. If you find a specific one that you can source somewhere (up to and include actually ordering from Digikey, probably not the optimal solution in Germany), feel free to post the datasheet here and we'll double-check it for you if you want. It's actually difficult with microcontrollers, because you might be operating the microcontroller at 5V, but the absolute maximum rating can be 6V (taking the ATMega8U2 as an example). So you need the TVS to be definitely completely open-circuit at 5V, but definitely completely closed-circuit at 6V. The part you originally specified won't actually do that, it doesn't start breaking down until 6.45V at least.
This is a good question, worthy of a new forum thread on its own: how do you protect a chip that nominally runs at 5V, but will be damaged by 6V? Because this sort of zener clamp isn't precise enough to do that. If you can run the micro at 3.3V instead, that'd be much easier.
If it doesn't interfere with the design, you should consider adding resistors on the test lines; that will limit the amount of current that can travel up through the protection diodes and into the VCC of the circuit. Otherwise, connecting a high-current 10V source to the pins will just blow up the TVS.