test: fix http-upgrade-agent flakiness

It's not guaranteed that the socket data is received in the same chunk
as the upgrade response. Listen for the `data` event to make sure all
the data is received.

test/parallel/test-http-upgrade-agent.js

@@ -46,9 +46,14 @@ srv.listen(common.PORT, '127.0.0.1', function() {
  req.end();
 
  req.on('upgrade', function(res, socket, upgradeHead) {
- // XXX: This test isn't fantastic, as it assumes that the entire response
- // from the server will arrive in a single data callback
- assert.equal(upgradeHead, 'nurtzo');
+ var recvData = upgradeHead;
+ socket.on('data', function(d) {
+ recvData += d;
+ });
+
+ socket.on('close', common.mustCall(function() {
+ assert.equal(recvData, 'nurtzo');
+ }));
 
  console.log(res.headers);
  var expectedHeaders = { 'hello': 'world',