test: use random ports where possible

This helps to prevent issues where a failed test can keep a bound
socket open long enough to cause other tests to fail with EADDRINUSE
because the same port number is used.

PR-URL: https://github.com/nodejs/node/pull/7045
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>

test/parallel/test-https-truncate.js

@@ -13,8 +13,6 @@ var fs = require('fs');
 var key = fs.readFileSync(common.fixturesDir + '/keys/agent1-key.pem');
 var cert = fs.readFileSync(common.fixturesDir + '/keys/agent1-cert.pem');
 
-var PORT = common.PORT;
-
 // number of bytes discovered empirically to trigger the bug
 var data = Buffer.allocUnsafe(1024 * 32 + 1);
 
@@ -29,8 +27,8 @@ function httpsTest() {
     server.close();
   });
 
-  server.listen(PORT, function() {
-    var opts = { port: PORT, rejectUnauthorized: false };
+  server.listen(0, function() {
+    var opts = { port: this.address().port, rejectUnauthorized: false };
     https.get(opts).on('response', function(res) {
       test(res);
     });