test: rename test-regress-nodejsGH-3238

Rename the test appropriately alongside mentioning the subsystem
Also, make a few basic changes to make sure the test conforms
to the standard test structure

Refs: nodejs#19105
Refs: https://github.com/nodejs/node/blob/master/doc/guides/writing-tests.md#test-structure

test/parallel/test-regress-GH-3238.js → test/parallel/test-cluster-kill-disconnect.js

@@ -1,5 +1,10 @@
 'use strict';
 const common = require('../common');
+
+// Check that cluster works perfectly for both `kill` and `disconnect` cases.
+// Also take into account that the `disconnect` event may be received after the
+// `exit` event.
+
 const assert = require('assert');
 const cluster = require('cluster');