selftests/net: Add icmp.sh for testing ICMP dummy address responses

This adds a new icmp.sh selftest for testing that the kernel will respond
correctly with an ICMP unreachable message with the dummy (192.0.0.8)
source address when there are no IPv4 addresses configured to use as source
addresses.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>