responder narrows subnets

This test checks to see if the responder accepts a more narrowed down
proposal as per https://tools.ietf.org/html/rfc5996#section-2.9

The initiator asks for a /24, all protocols and ports.
The responder only allows a /28, all protocols and ports.

Because narrowing=yes on the responder, it will allow and suggest its
narrowed proposal. Because the initiator has narrowing=yes, it should
accept the narrowed proposal and install it.

