3.6.2 Test nonce generation

There's two ways which this could be tested.

1) Preprocess not taking in an arbitrary RNG item, yet the relevant bytes

This would be an unsafe level of refactoring, in my opinion.

2) Test random_nonce and test the passed in RNG eventually ends up at
random_nonce.

This takes the latter route, both verifying random_nonce meets the vectors
and that the FROST machine calls random_nonce properly.

crypto/ciphersuite/src/lib.rs

@@ -90,9 +90,7 @@ pub trait Ciphersuite: Clone + Copy + PartialEq + Eq + Debug + Zeroize {
     // ff mandates this is canonical
     let res = Option::<Self::F>::from(Self::F::from_repr(encoding))
       .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "non-canonical scalar"));
-    for b in encoding.as_mut() {
-      b.zeroize();
-    }
+    encoding.as_mut().zeroize();
     res
   }