Tighten up test assertions after review

Signed-off-by: Tim Perry <pimterry@gmail.com>

Author: pimterry

test/parallel/test-http-client-request-listeners-leak.js

@@ -1,5 +1,6 @@
 'use strict';
 const common = require('../common');
+const assert = require('assert');
 const http = require('http');
 const { defaultMaxListeners } = require('events');
 
@@ -16,6 +17,14 @@ server.listen(0, common.mustCall(() => {
   const agent = new http.Agent({ keepAlive: true });
   const port = server.address().port;
 
+  // Count actual socket creations to confirm reuse:
+  let createSocketCount = 0;
+  const origCreateSocket = agent.createSocket.bind(agent);
+  agent.createSocket = function(...args) {
+    createSocketCount++;
+    return origCreateSocket(...args);
+  };
+
   function executeHttpGet() {
     return new Promise((resolve) => {
       const req = http.get({ host: '127.0.0.1', port, agent });
@@ -30,9 +39,12 @@ server.listen(0, common.mustCall(() => {
     for (let i = 0; i < defaultMaxListeners + 1; i++) {
       await executeHttpGet();
     }
+
+    assert.strictEqual(createSocketCount, 1);
+
     server.close();
     agent.destroy();
   }
 
-  main();
+  main().then(common.mustCall());
 }));