testing: Add scenarios that use a CA with two intermediate CA certificates

Mainly to test TKM's ability for handling multiple CAs and that the
received intermediate CA certificates are passed in the right order.
But also added a regular scenario where two intermediate CA certificates
are sent by one of the clients.

testing/tests/tkm/multi-level-ca/hosts/dave/etc/swanctl/swanctl.conf

@@ -0,0 +1,25 @@
+connections {
+
+  host-host {
+    local_addrs  = PH_IP_DAVE
+    remote_addrs = PH_IP_MOON
+
+    proposals = aes256-sha512-modp4096
+
+    local {
+      auth = pubkey
+      certs = daveCert.pem
+      id = dave@strongswan.org
+    }
+    remote {
+      auth = pubkey
+      id = moon.strongswan.org
+    }
+    children {
+      host-host {
+        mode = transport
+        esp_proposals = aes256-sha512-modp4096
+      }
+    }
+  }
+}