IKEv2 connection through NAT that expires; liveness=EAST

This test demonstrates how only having liveness enabled on EAST is
broken - any outbound traffic from ROAD (behind the NAT) fools EAST
into thinking that the connection is still up.

- ROAD establishes with EAST, a ping is exchanged.

- to simulate the expire, NIC changes its NAT mapping.

- ROAD tries to send a further ping but, as expected, gets no response:

  With Linux, the failure is on the return path!

  Even though the kernel's states are configured for the OLD port it
  accepts the packet from the NEW PORT.  The ping reply goes back to
  the old PORT which the NAT drops.

- the broken traffic flow is verified on EAST and NIC

- EAST sees the inbound the and punts liveness

- time passes

- since EAST sees no further inbound traffic liveness is initiated

  Since the NAT mapping changed, the liveness request can't reach ROAD
  and, hence, times out.

  Since ROAD can't see traffic from EAST and isn't doing liveness it
  is left hanging (which is why mobike={yes,no,nat} has no effect).
