selftests/net: test tcp connection load balancing

selftests/net: test tcp connection load balancing

Verify that TCP connections use both routes when connecting multiple
times to a remote service over a two nexthop multipath route.

Use socat to create the connections. Use tc prio + tc filter to
count routes taken, counting SYN packets across the two egress
devices. Also verify that the saddr matches that of the device.

To avoid flaky tests when testing inherently randomized behavior,
set a low bar and pass if even a single SYN is observed on each
device.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250424143549.669426-4-willemdebruijn.kernel@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>