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
and should not detect a dead peer.

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.
4) West blocks packet flow
5) East detects dead pear and takes dpdaction=clear
6) West unblocks packet flow, tries ping which fails
