selftests: drv-net: xdp: Add a single-buffer XDP_TX test.

Test single-buffer XDP_TX for packets with various payload sizes.
Update the socat TX command to generate packets with 0 length payloads.

Signed-off-by: Dimitri Daskalakis <dimitri.daskalakis1@gmail.com>
Link: https://patch.msgid.link/20250821014023.1481662-3-dimitri.daskalakis1@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>