Add unit tests that check for common RNG failure modes

Check that crypto_rand doesn't return all zeroes, identical values,
or incrementing values (OpenSSL's rand_predictable feature).

changes/rand-failure-modes

@@ -0,0 +1,5 @@
+  o Minor features (unit tests, random number generation):
+    - Add unit tests that check for common RNG failure modes, such as
+      returning all zeroes, identical values, or incrementing values
+      (OpenSSL's rand_predictable feature).
+      Patch by "teor".