selftests: expand txtimestamp with ipv6 dgram + raw and pf_packet

Expand the transmit timestamp regression test with support for
missing protocols: ipv6 datagram and raw and pf_packet.

Also refine resolve_hostname to independently request AF_INET or
AF_INET6 addresses. Else, ipv4 addresses may be returned as AF_INET6.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>