This tests the RFC3706 DPD implementation, ensuring we only enable it
when both sides support it. Note that we don't have a way for _not_ supporting
or announcing it. So west and east support DPD, but only east activates it. So
only probes from east to west (and their replies) should happen. West should
not send out R_U_THERE requests.

Order of Operations:

1) East sets up for tunnel with DPD support.
2) West initiates tunnel to east, but west does not have DPD enabled.
3) West sends icmp packets to east, which should work.