test: add env to failure message

PR-URL: https://github.com/nodejs/node/pull/15816
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-child-process-default-options.js

@@ -45,5 +45,6 @@ child.stdout.on('data', function(chunk) {
 
 process.on('exit', function() {
   assert.ok(response.includes('HELLO=WORLD'),
-            'spawn did not use process.env as default');
+            'spawn did not use process.env as default' +
+            `(process.env.HELLO = ${process.env.HELLO})`);
 });