test: use shorthand properties

PR-URL: https://github.com/nodejs/node/pull/18105
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http-client-read-in-error.js

@@ -34,7 +34,7 @@ class Agent extends http.Agent {
 const agent = new Agent();
 
 http.request({
-  agent: agent
+  agent
 }).once('error', function() {
   console.log('ignore');
 });