test sec_label narrowing when subnet=0.0.0.0/0 and childless

When creating the child, the code needs to instantiate and narrow the
IKE SA's hybrid sec_label template-instance connection.

Because narrowing was required, the code was instead rejecting the
connection.

(as an aside I suspect all the effort to find a better connection
matching the traffic selectors is pointless here - it can't switch and
instead should always instantiate the IKE SA's connection)
