test/system: skip "IPv6 default address assignment"

It is not working in all ipv6 setups. In the new lima testing env
default_addr is just null causing the test to fail because the container
address was "fec0::5055:55ff:fe2b:6cc9".

For now skip this until we can figure something better out.

Signed-off-by: Paul Holzinger <pholzing@redhat.com>