test: remove modifcation to common.PORT

A possibly-buggy fixture server uses `common.PORT+1000` for its port
rather than `common.PORT`. That could result in it clashing with other
ports if tests are run in parallel. The test runner increments
`common.PORT` by 100 for each running instance for tests. Change to use
common.PORT and have the tests that use the fixture start with
common.PORT+1 for anything they need.

PR-URL: #6990
Refs: #6989
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

test/fixtures/clustered-server/app.js

@@ -31,5 +31,5 @@ if (cluster.isMaster) {
   }
 } else {
   var server = http.createServer(handleRequest);
-  server.listen(common.PORT+1000);
+  server.listen(common.PORT);
 }

test/parallel/test-debug-port-cluster.js

@@ -3,7 +3,7 @@ const common = require('../common');
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
-const PORT_MIN = common.PORT;
+const PORT_MIN = common.PORT + 1;  // The fixture uses common.PORT.
 const PORT_MAX = PORT_MIN + 2;
 
 const args = [

test/parallel/test-debug-signal-cluster.js

@@ -3,7 +3,7 @@ var common = require('../common');
 var assert = require('assert');
 var spawn = require('child_process').spawn;
 
-var port = common.PORT + 42;
+var port = common.PORT + 1;  // The fixture uses common.PORT.
 var args = ['--debug-port=' + port,
             common.fixturesDir + '/clustered-server/app.js'];
 var options = { stdio: ['inherit', 'inherit', 'pipe', 'ipc'] };