Minimal support for IPv6 upstreams in IpServer tests.

This makes the IpServer tests a bit more realistic. It is needed
by an upcoming change that adds new functionality, and is in its
own commit because it modifies the behaviour of all the existing
tests.

Test: atest TetheringTests
Change-Id: I8c3267bea983d1453c010b7b6b85548b9481a153