test: add a test description

PR-URL: https://github.com/nodejs/node/pull/16833
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

test/parallel/test-cluster-fork-env.js

@@ -21,6 +21,11 @@
 
 'use strict';
 require('../common');
+
+// This test checks that arguments provided to cluster.fork() will create
+// new environment variables and override existing environment variables
+// in the created worker process.
+
 const assert = require('assert');
 const cluster = require('cluster');