Test |BN_uadd| and |BN_usub|.

Also, update the documentation about aliasing for |BN_usub|. It might
be better to find a way to factor out the shared logic between the
tests of these functions and the tests of |BN_add| and |BN_usub|, but
doing so would end up up creating a lot of parameters due to the many
distinct strings used in the messages.

Change-Id: Ic9d714858212fc92aa6bbcc3959576fe6bbf58c3
Reviewed-on: https://boringssl-review.googlesource.com/8593
Reviewed-by: David Benjamin <davidben@google.com>

include/openssl/bn.h

diff --git a/include/openssl/bn.h b/include/openssl/bn.h
index bc715b0..23dd807 100644
--- a/include/openssl/bn.h
+++ b/include/openssl/bn.h
@@ -388,7 +388,7 @@
 OPENSSL_EXPORT int BN_sub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b);
 
 /* BN_usub sets |r| = |a| - |b|, where |a| and |b| are non-negative integers,
- * |b| < |a| and |r| must be a distinct pointer from |a| and |b|. It returns
+ * |b| < |a| and |r| may be the same pointer as either |a| or |b|. It returns
  * one on success and zero on allocation failure. */
 OPENSSL_EXPORT int BN_usub(BIGNUM *r, const BIGNUM *a, const BIGNUM *b);