selftests: net: fix file owner for broadcast_ether_dst test

selftests: net: fix file owner for broadcast_ether_dst test

Ensure the output file is always owned by root (even if tcpdump was
compiled with `--with-user`), by passing the `-Z root` argument when
invoking it.

Cc: stable@vger.kernel.org
Reported-by: Edoardo Canepa <edoardo.canepa@canonical.com>
Closes: https://bugs.launchpad.net/ubuntu-kernel-tests/+bug/2129815
Fixes: bf59028ea8d4 ("selftests: net: add test for destination in broadcast packets")
Suggested-by: Edoardo Canepa <edoardo.canepa@canonical.com>
Tested-by: Ross Porter <ross.porter@canonical.com>
Signed-off-by: Ross Porter <ross.porter@canonical.com>
Link: https://patch.msgid.link/20260617061039.79717-2-ross.porter@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>