selftests: drv-net: add configs for the TSO test

Add missing config options for the tso.py test, specifically
to make sure the kernel is built with vxlan and gre tunnels.

I noticed this while adding a TSO-capable device QEMU to the CI.
Previously we only run virtio tests and it doesn't report LSO
stats on the QEMU we have.

Fixes: 0d0f4174 ("selftests: drv-net: add a simple TSO test")
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250604001653.853008-1-kuba@kernel.org


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