selftests: drv-net: gro: add 1 byte payload test

selftests: drv-net: gro: add 1 byte payload test

Small IPv4 packets get padded to 60B, this may break / confuse
some buggy implementations. Add a test to coalesce a 1B payload.
Keep this separate from the lrg_sml test because I suspect some
implementations may not handle this case (treat padded frames
as ineligible for coalescing).

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260402210000.1512696-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>