Add expect-connection guest image script

This script can be used in pretest.dat files to wait until an IPsec
connection becomes available. This avoids unconditional sleeps and
improves test performance.

The ipv6 tests have been updated to use the expect-connection script.

testing/tests/ipv6/net2net-ikev1/pretest.dat

@@ -6,6 +6,6 @@ sun::"ip route add fec1:\:/16 via fec0:\:1"
 bob::"ip route add fec1:\:/16 via fec2:\:1"
 moon::ipsec start
 sun::ipsec start
-moon::sleep 2 
+moon::expect-connection net-net
+sun::expect-connection net-net
 moon::ipsec up net-net
-moon::sleep 1