testing: Use alternative approach for retransmits in ikev1/dpd-restart scenario

With a long delay, the retransmit might not get sent before further tests
are evaluated on faster machines, while more retransmits should still allow
the scenario to succeed on slower ones.

testing/tests/ikev1/dpd-restart/hosts/carol/etc/strongswan.conf

@@ -2,5 +2,5 @@
 
 charon {
   load = random nonce openssl pem pkcs1 curl revocation vici kernel-netlink socket-default
-  retransmit_timeout = 4.0
+  retransmit_tries = 6
 }