testing: Enable mgf1 plugin for scenarios where FreeRADIUS uses PSS signatures

Looks like a cipher suite without DHE was selected previously.

Could be a side-effect of dc1085734f ("testing: Remove unnecessary
FreeRADIUS dh_file option as recommended in the log").

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

@@ -1,7 +1,7 @@
 # /etc/strongswan.conf - strongSwan configuration file
 
 charon {
-  load = random nonce aes md5 sha1 sha2 pem pkcs1 curve25519 gmp x509 curl revocation hmac kdf stroke kernel-netlink socket-default eap-identity eap-md5 eap-ttls updown
+  load = random nonce aes md5 sha1 sha2 pem pkcs1 curve25519 gmp x509 curl revocation hmac kdf mgf1 stroke kernel-netlink socket-default eap-identity eap-md5 eap-ttls updown
   multiple_authentication=no
   syslog {
     daemon {