A routed subnet-to-subnet tunnel using xfrmi interface.

xfrmi need a kernel support.
The support is available in kernel 4.19+ with CONFIG_XFRM_INTERFACE=y | m

Because this test is subnet-to-subnet and the other end is not inside the
'subnet' we do not need any specific 'ip rule' lookups here.
Just add route to the subnet via dev ipsec0

Also note output marks are not necessary, there will not be any routing loop,
because that end IP address is outside the extruded subnet.

This also tests for refcounting added connection that are re-added without
having gotten up. This is to confirm this error no longer happens:

002 "north": cannot delete ipsec-interface=ipsec1 if_id=1, not created by pluto

