I would very frequently use an unplugged cable as an interview test, but never had an automated one.

Just a couple of ideas, presuming your wires can be short (a metre or two)

* Use a switch as in that video -- any switch which interrupts both wires of a pair will cause faults.  Endless variety on ebay ("dpdt toggle switch latching screw terminal"), just chop a patch cable and wire it up.
* Use a network controlled relay for scripted control.  I see 16-relay units on ebay ("ethernet relay") for about £25 with screw-terminals, wire same as switch.  You normally control it with web fetches, manually or by `curl` or similar.

Alternatively you have an old hub you can power that up and down, perhaps with a remote controlled power strip (eg from APC).  Hubs won't be visible in CDP or similar.

I'd do it with the relay as you'll be able to generate interesting faults.