test: update test-https-server-keep-alive-timeout

PR-URL: https://github.com/nodejs/node/pull/15805
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

test/parallel/test-https-server-keep-alive-timeout.js

@@ -1,18 +1,19 @@
 'use strict';
 
 const common = require('../common');
+
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+const fixtures = require('../common/fixtures');
 const https = require('https');
 const tls = require('tls');
-const fs = require('fs');
 
 const tests = [];
 
 const serverOptions = {
-  key: fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`),
-  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`)
+  key: fixtures.readKey('agent1-key.pem'),
+  cert: fixtures.readKey('agent1-cert.pem')
 };
 
 function test(fn) {