Extend the existing test to exercise UDP GSO egress through devices with
various offload capabilities, including lack of checksum offload, which is
the default case for TUN/TAP devices.

Test against a dummy device because it is simpler to set up then TUN/TAP.
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240626-linux-udpgso-v2-2-422dfcbd6b48@cloudflare.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>