testing: Load kdf plugin in all scenarios that require it

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

testing/tests/ikev2/rw-eap-ttls-only/hosts/dave/etc/strongswan.conf

@@ -5,7 +5,7 @@ swanctl {
 }
 
 charon-systemd {
-  load = random nonce aes md5 sha1 sha2 pem pkcs1 curve25519 mgf1 gmp x509 curl revocation hmac gcm vici kernel-netlink socket-default eap-identity eap-md5 eap-ttls updown
+  load = random nonce aes md5 sha1 sha2 pem pkcs1 curve25519 mgf1 gmp x509 curl revocation hmac kdf gcm vici kernel-netlink socket-default eap-identity eap-md5 eap-ttls updown
 
   multiple_authentication=no
   syslog {