Add tests for BN_nnmod.

Change-Id: Ic72e00bb01d254408671b3f8d036be3cd4c06086
Reviewed-on: https://boringssl-review.googlesource.com/8606
Reviewed-by: David Benjamin <davidben@google.com>

crypto/bn/bn_test.cc

diff --git a/crypto/bn/bn_test.cc b/crypto/bn/bn_test.cc
index 53de206..e12146e 100644
--- a/crypto/bn/bn_test.cc
+++ b/crypto/bn/bn_test.cc
@@ -577,6 +577,20 @@
     }
   }
 
+  // Test BN_nnmod.
+  if (!BN_is_negative(b.get())) {
+    ScopedBIGNUM nnmod(BN_new());
+    if (!nnmod ||
+        !BN_copy(nnmod.get(), remainder.get()) ||
+        (BN_is_negative(nnmod.get()) &&
+         !BN_add(nnmod.get(), nnmod.get(), b.get())) ||
+        !BN_nnmod(ret.get(), a.get(), b.get(), ctx) ||
+        !ExpectBIGNUMsEqual(t, "A % B (non-negative)", nnmod.get(),
+                            ret.get())) {
+      return false;
+    }
+  }
+
   return true;
 }