static Kem dHKemX25519HkdfSha256(UnsafeDeterminsticKeyPairOnlyUsedByTesting testingKP) {
        return new DHKemX25519HkdfSha256(() -> testingKP.keyPair);
    }
Returns an unsafe test KEM that returns a single fixed, deterministic key pair.

As the name implies, this must only ever be used in the context of testing. If anyone tries
to be clever and use this anywhere else, I will find them and bite them in the ankles!
security-utils/src/main/java/com/yahoo/security/hpke/Kem.java