benchmark,windows: TCP.readStart() meaningful only after completion

fixes #11972

PR-URL: https://github.com/nodejs/node/pull/12258
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

benchmark/net/tcp-raw-pipe.js

@@ -97,8 +97,6 @@ function client() {
   if (err)
     fail(err, 'connect');
 
-  clientHandle.readStart();
-
   clientHandle.onread = function(nread, buffer) {
     if (nread < 0)
       fail(nread, 'read');
@@ -112,6 +110,8 @@ function client() {
 
     bench.start();
 
+    clientHandle.readStart();
+
     setTimeout(function() {
       // multiply by 2 since we're sending it first one way
       // then then back again.