add test for shared secrets to mkem_kex()

[Sashan]

There is a TODO note in fuzz/ml-kem.c, it reads as follows:

405 static void mlkem_kex(uint8_t **buf, size_t *len, void *key1, void *key2,
406                       void **out1, void **out2)
407 {
408     EVP_PKEY *alice = (EVP_PKEY *)key1;
409     EVP_PKEY *bob = (EVP_PKEY *)key2;
410     size_t boblen, alicelen;
411     uint8_t *bobshare = NULL;
412     uint8_t *aliceshare = NULL;
413
414     do_derive(alice, bob, &aliceshare, &alicelen);
415     do_derive(bob, alice, &bobshare, &boblen);
416
417     /*
418      * TODO add check of shared secrets here when ML-KEM supports this
419      */
420     OPENSSL_free(bobshare);
421     OPENSSL_free(aliceshare);
422 }

I'm just filing the ticket to remember