test: add logging for test-debug-port-cluster

The test is currently flaky and CI provides no real information because
the test times out rather than failing on an assertion. Add logging to
gather more information about the failure.

Refs: nodejs#6754

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

@@ -1,12 +1,12 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
-var spawn = require('child_process').spawn;
+const common = require('../common');
+const assert = require('assert');
+const spawn = require('child_process').spawn;
 
 const PORT_MIN = common.PORT + 1337;
 const PORT_MAX = PORT_MIN + 2;
 
-var args = [
+const args = [
   '--debug=' + PORT_MIN,
   common.fixturesDir + '/clustered-server/app.js'
 ];
@@ -16,12 +16,12 @@ child.stderr.setEncoding('utf8');
 
 let stderr = '';
 child.stderr.on('data', (data) => {
+  process.stderr.write(`[DATA] ${data}`);
   stderr += data;
-  if (child.killed !== true && stderr.includes('all workers are running'))
+  if (child.killed !== true && stderr.includes('all workers are running')) {
     child.kill();
-});
-
-process.on('exit', () => {
-  for (let port = PORT_MIN; port <= PORT_MAX; port += 1)
-    assert(stderr.includes(`Debugger listening on port ${port}`));
+    for (let port = PORT_MIN; port <= PORT_MAX; port += 1) {
+      assert(stderr.includes(`Debugger listening on port ${port}`));
+    }
+  }
 });