selftests: add few test cases for tap driver

Few test cases related to the fix for 924a9bc3

:
"net: check if protocol extracted by virtio_net_hdr_set_proto is correct"

Need test for the case when a non-standard packet (GSO without NEEDS_CSUM)
sent to the tap device causes a BUG check in the tap driver.

Signed-off-by: Cezar Bulinaru <cbulinaru@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>