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/posttest.dat

@@ -0,0 +1,7 @@
+carol::swanctl --terminate --ike home 2> /dev/null
+dave::swanctl --terminate --ike home 2> /dev/null
+carol::systemctl stop strongswan
+dave::systemctl stop strongswan
+moon::systemctl stop strongswan
+carol::cd /etc/swanctl; rm -r rsa/* x509/* x509ca/*
+moon::cd /etc/swanctl; rm -r rsa/* x509/* x509ca/*