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/two-certs/pretest.dat

@@ -2,7 +2,7 @@ moon::iptables-restore < /etc/iptables.rules
 carol::iptables-restore < /etc/iptables.rules
 moon::ipsec start
 carol::ipsec start
-carol::sleep 1
+carol::expect-connection alice
+carol::expect-connection venus
 carol::ipsec up alice
 carol::ipsec up venus
-carol::sleep 1