fixup: move test, add comment

Author: Trott

test/parallel/test-cluster-shared-worker-added-after-disconnect.js renamed to test/parallel/test-cluster-shared-leak.js

@@ -1,3 +1,6 @@
+// In Node 4.2.1, this test triggers an assertion in cluster.js. The assertion
+// protects against memory leaks. https://github.com/nodejs/node/pull/3510
+
 'use strict';
 const common = require('../common');
 const assert = require('assert');