selftests: drv-net: adjust to socat changes

selftests: drv-net: adjust to socat changes

socat v1.8.1.0 now defaults to shut-null, it sends an extra
0-length UDP packet when sender disconnects. This breaks
our tests which expect the exact packet sequence.

Add shut-none which was the old default where necessary.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Reviewed-by: Joe Damato <joe@dama.to>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20260404230103.2719103-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>