selftests: net: veth: test syncing GRO and XDP state while device is down

Test that we keep GRO flag in sync when XDP is disabled while
the device is closed.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>