test: replace common.fixtures with fixtures module

PR-URL: https://github.com/nodejs/node/pull/15877
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

test/parallel/test-https-agent-sockets-leak.js

@@ -4,14 +4,14 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
-const fs = require('fs');
 const https = require('https');
 const assert = require('assert');
+const fixtures = require('../common/fixtures');
 
 const options = {
-  key: fs.readFileSync(`${common.fixturesDir}/keys/agent1-key.pem`),
-  cert: fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`),
-  ca: fs.readFileSync(`${common.fixturesDir}/keys/ca1-cert.pem`)
+  key: fixtures.readKey('agent1-key.pem'),
+  cert: fixtures.readKey('agent1-cert.pem'),
+  ca: fixtures.readKey('ca1-cert.pem')
 };
 
 const server = https.Server(options, common.mustCall((req, res) => {