Test CREATE_CHILD_SA with east's pfs=yes ms_dh_downgrade=yes

Wes tries pfs={yes,no} X ms_dh_downgrade={yes,no}.

All but pfs=yes,ms_dh_downgrade=no should interop.

XXX: the failing case cascades causing the next interop to hang.  See
ikev2-child-00-double-hang.
