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/swanctl/multi-level-ca-l3/pretest.dat

@@ -0,0 +1,10 @@
+moon::systemctl start strongswan
+carol::systemctl start strongswan
+dave::systemctl start strongswan
+moon::expect-connection research
+carol::expect-connection alice
+carol::swanctl --initiate --child alice 2> /dev/null
+carol::swanctl --initiate --child venus 2> /dev/null
+dave::expect-connection alice
+dave::swanctl --initiate --child alice 2> /dev/null
+dave::swanctl --initiate --child venus 2> /dev/null