testing: Globally define logging via syslog for charon-systemd

We could make the same change for charon (actually setting it for charon
in strongswan.conf.testing would work for charon-systemd too), however,
there are dozens of test cases that currently set charondebug in
ipsec.conf.

testing/hosts/default/etc/strongswan.conf.testing

@@ -8,5 +8,14 @@ charon {
     }
   }
 }
+charon-systemd {
+  syslog {
+    daemon {
+    }
+    auth {
+      default = 0
+    }
+  }
+}
 
 include strongswan.conf