selftests: traceroute: Test traceroute with different source IPs

When generating ICMP error messages, the kernel will prefer a source IP
that is on the same subnet as the destination IP (see
inet_select_addr()). Test this behavior by invoking traceroute with
different source IPs and checking that the ICMP error message is
generated with a source IP in the same subnet.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250908073238.119240-8-idosch@nvidia.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>