testing: Add a base strongswan.conf file used by all hosts in all scenarios

We will use this to set some defaults (e.g. timeouts to make testing
negative tests quicker).  We don't want these settings to show up in the
configs of the actual scenarios though.

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

@@ -0,0 +1 @@
+include strongswan.conf

testing/scripts/recipes/013_strongswan.mk

@@ -18,6 +18,7 @@ NUM_CPUS := $(shell getconf _NPROCESSORS_ONLN)
 
 CONFIG_OPTS = \
 	--sysconfdir=/etc \
+	--with-strongswan-conf=/etc/strongswan.conf.testing \
 	--with-random-device=/dev/urandom \
 	--disable-load-warning \
 	--enable-curl \