Corrupt the newhope secret key directly.

Rather than use an internal function in a test (which would need an
OPENSSL_EXPORT to work in a shared-library build), this change corrupts
the secret key directly.

Change-Id: Iee501910b23a0affaa0639dcc773d6ea2d0c5a82
Reviewed-on: https://boringssl-review.googlesource.com/7780
Reviewed-by: Matt Braithwaite <mab@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/newhope/newhope_test.c

@@ -77,8 +77,8 @@ static int test_invalid_sk_a() {
       return 0;
     }
 
-    /* Overwrite the secret key */
-    newhope_poly_getnoise(sk);
+    /* Corrupt the secret key */
+    NEWHOPE_keygen(servermsg /* not used below */, sk);
 
     /* Alice uses Bob's response to get her secret key */
     if (!NEWHOPE_server_compute_key(server_key, sk, clientmsg,