selftests/net: Add test coverage for UDP GSO software fallback

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