test: add NODE_UNIQUE_ID value to err message

PR-URL: #15914
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-cluster-basic.js

@@ -5,7 +5,8 @@ const assert = require('assert');
 const cluster = require('cluster');
 
 assert.strictEqual('NODE_UNIQUE_ID' in process.env, false,
-                   'NODE_UNIQUE_ID should be removed on startup');
+                   `NODE_UNIQUE_ID (${process.env.NODE_UNIQUE_ID})` +
+                   'should be removed on startup');
 
 function forEach(obj, fn) {
   Object.keys(obj).forEach((name, index) => {