testing: Make sure ML-KEM scenarios use our ml plugin

testing: Make sure ML-KEM scenarios use our ml plugin

We now support OpenSSL's implementation in the openssl plugin.  This
makes sure our plugin is used on at least one of the hosts if we ever
switch to an OpenSSL version that supports ML-KEM.

In the ikev2/rw-mlkem scenario the logic is reversed.  There the ml plugin
is preferred on moon to test the responder side (and carol for the
initiator) and dave will switch to OpenSSL if it ever provides ML-KEM.