Ethernet mechanical switch for testing
You may try a simple mechanical relay in the middle of the cable.

If your purpose is connect/disconnect testing, you don't need a high-speed control, but a high-speed connection. A mechanical relay allows that, since it's only a metallic connection that won't interfere much (except for geometry EMI).

Granted, you'll have to find a relay that doesn't interfere too much on the Ethernet signal, but you probably won't have too many problems unless you're doing Gigabit or faster. In this case, I'd try to find the smallest relay possible, since this tends to keep EMI to higher frequencies (and Ethernet is not a high-power connection).