test: raise pummel/test-net-throttle write req size

pummel/test-net-throttle assumes that a couple of big write requests result in
some of them getting queued because the kernel's send buffer fills up.

Said assumption breaks on systems with large send buffers. Raise the size of
the write request to ameliorate the issue.

This is a back-port of commit 6770555 from the master branch.

test/pummel/test-net-throttle.js

@@ -26,7 +26,7 @@ var common = require('../common');
 var assert = require('assert');
 var net = require('net');
 
-var N = 160 * 1024;
+var N = 1024 * 1024;
 var part_N = N / 3;
 var chars_recved = 0;
 var npauses = 0;