test: fix pummel/test-net-connect-memleak

* Run the garbage collector before creating the big array. It doesn't
  matter now but if in the future something in node.js core creates
  a lot of reclaimable garbage, that will break the test's expectation.

* The first RSS check was being done too late. The garbage collector
  might have run before the check, throwing off the 'reclaimed memory'
  calculation.

* Due to changes in how V8 represents the big array internally, the
  actual memory usage is just below 256 MB on x64. Update the test's
  expectation.

test/pummel/test-net-connect-memleak.js

@@ -28,9 +28,12 @@ var net = require('net');
 assert(typeof gc === 'function', 'Run this test with --expose-gc');
 net.createServer(function() {}).listen(common.PORT);
 
+var before = 0;
 (function() {
   // 2**26 == 64M entries
+  gc();
   for (var i = 0, junk = [0]; i < 26; ++i) junk = junk.concat(junk);
+  before = process.memoryUsage().rss;
 
   net.createConnection(common.PORT, '127.0.0.1', function() {
     assert(junk.length != 0);  // keep reference alive
@@ -40,11 +43,10 @@ net.createServer(function() {}).listen(common.PORT);
 })();
 
 function done() {
-  var before = process.memoryUsage().rss;
   gc();
   var after = process.memoryUsage().rss;
   var reclaimed = (before - after) / 1024;
   console.log('%d kB reclaimed', reclaimed);
-  assert(reclaimed > 256 * 1024);  // it's more like 512M on x64
+  assert(reclaimed > 128 * 1024);  // It's around 256 MB on x64.
   process.exit();
 }