sw test too: use assured Forwarding behavior DSCP value for dscp_remark

OpenFlow spec describes "DSCP values that do not encode a drop
precedence are not modified".

Diffserv Assured Forwarding described in rfc2597(+rfc3260) has
DSCP values which encodes "drop precedence".

I think we'd better test OFPMBT_DSCP_REMARK works on AF21 -> AF22
rating, for example.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>