test-network: include IPv6 tests unconditionally

Previously we would incorrectly detect that ipv6 isn't supported if the ping binary isn't present (as it may not be in a relatively stripped down container image), or if ICMP packets aren't permitted (as they often aren't in containers).

We don't really have a need to run these network tests on non-IPv6 systems, so it makes more sense to just run them unconditionally.