Added testing for IPv6 to peer grouper tests

So far the peer grouper only tests IPv4. As part of the IPv6 enabling the peer grouper testing now runs all tests for IPv6 too.
I'm sorry but I don't like that the IPv4 and IPv6 tests are almost identical. This creates unnecessary maintenance burden when these tests change later on. Extra care will need to be taken to ensure the IPv4 and IPv6 tests stay in sync.

What about making desiredPeerGroupTests hold functions which take a single argument (the IP type) and return the test definition struct? TestDesiredPeerGroup would then call each func twice - once with IPv4 and once with IPv6 running the test defined by each call.