Merge #1378: ellswift: fix probabilistic test failure when swapping sides

c424e2f ellswift: fix probabilistic test failure when swapping sides (Jonas Nick)

Pull request description:

  Reported by jonatack in bitcoin/bitcoin#28079.

  When configured with `--disable-module-ecdh --enable-module-recovery`, then `./tests  64 81af32fd7ab8c9cbc2e62a689f642106` fails with
  ```
  src/modules/ellswift/tests_impl.h:396: test condition failed: secp256k1_memcmp_var(share32_bad, share32a, 32) != 0
  ```

  This tests verifies that changing the `party` bit of the `secp256k1_ellswift_xdh` function results in a different share. However, that's not the case when the secret keys of both parties are the same and this is actually what happens in the observed test failure. The keys can be equal in this test case because they are created by the `random_scalar_order_test` function whose output is not uniformly random (it's biased towards 0).

  This commit restores the assumption that the secret keys differ.

ACKs for top commit:
  sipa:
    utACK c424e2f
  real-or-random:
    utACK c424e2f

Tree-SHA512: d1ab61473a77478f9aeffb21ad73e0bba478c90d8573c72ec89d2e0140434cc65c9d5f4d56e5f259931dc68fc1800695c6cd5d63d9cfce4c1c4d6744eeaa2028

Author: real-or-random

src/modules/ellswift/tests_impl.h

@@ -322,7 +322,9 @@ void run_ellswift_tests(void) {
         secp256k1_testrand256_test(auxrnd32a);
         secp256k1_testrand256_test(auxrnd32b);
         random_scalar_order_test(&seca);
-        random_scalar_order_test(&secb);
+        /* Draw secb uniformly at random to make sure that the secret keys
+         * differ */
+        random_scalar_order(&secb);
         secp256k1_scalar_get_b32(sec32a, &seca);
         secp256k1_scalar_get_b32(sec32b, &secb);