testing: Remove nearly all sleep calls from pretest and posttest scripts

By consistently using the `expect-connection` helper we can avoid pretty
much all previously needed calls to sleep.

testing/tests/ikev2/double-nat/pretest.dat

@@ -6,6 +6,5 @@ sun::iptables -t nat -A POSTROUTING -o eth0 -s 10.2.0.0/16 -p tcp -j SNAT --to-s
 sun::iptables -t nat -A PREROUTING -i eth0 -s PH_IP_MOON -p udp -j DNAT --to-destination PH_IP_BOB
 alice::ipsec start
 bob::ipsec start
-alice::sleep 2
+alice::expect-connection nat-t
 alice::ipsec up nat-t
-alice::sleep 1