Add sack tx unit test

Change-Id: Ib91db6e531231bdc52b0104673a912bee024872f
Signed-off-by: Florin Coras <fcoras@cisco.com>
diff --git a/src/vnet/tcp/tcp_format.c b/src/vnet/tcp/tcp_format.c
index 1ca2f58..3148fd4 100644
--- a/src/vnet/tcp/tcp_format.c
+++ b/src/vnet/tcp/tcp_format.c
@@ -128,6 +128,18 @@
   return s;
 }
 
+u8 *
+format_tcp_sacks (u8 * s, va_list * args)
+{
+  sack_block_t *sacks = va_arg (*args, sack_block_t *);
+  sack_block_t *block;
+  vec_foreach (block, sacks)
+  {
+    s = format (s, " start %u end %u\n", block->start, block->end);
+  }
+  return s;
+}
+
 /*
  * fd.io coding-style-patch-verification: ON
  *