Make the socketDestroy tests use a fake tun interface.

On real devices, we often don't want to destroy sockets that
are on loopback. This CL makes our integration tests use
sockets that don't look like they're on loopback, making it
possible to test code that does not destroy sockets that are
on loopback.

Bug: 30186506
Change-Id: I1ea56f069f528f25dddd5898b2269a638318f820