testing: Load kdf plugin in all scenarios that require it

Once we use plugin-provided prf+() these won't work otherwise.

testing/tests/route-based/net2net-gre/hosts/sun/etc/strongswan.conf

@@ -5,7 +5,7 @@ swanctl {
 }
 
 charon-systemd {
-  load = random nonce aes sha1 sha2 pem pkcs1 curve25519 gmp x509 curl revocation hmac vici kernel-netlink socket-default updown
+  load = random nonce aes sha1 sha2 pem pkcs1 curve25519 gmp x509 curl revocation hmac kdf vici kernel-netlink socket-default updown
 }
 
 charon {