test: use fixtures in test-https-localaddress.js

PR-URL: #15811
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@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-localaddress.js

@@ -6,13 +6,13 @@ if (!common.hasCrypto)
 if (!common.hasMultiLocalhost())
   common.skip('platform-specific test.');
 
-const fs = require('fs');
+const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const https = require('https');
 
 const options = {
-  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')
 };
 
 const server = https.createServer(options, function(req, res) {