test: use fixtures.readKey in https-timeout-server

Replace `common.fixturesDir` usage in test-https-timeout-server.js with
`fixtures.readKey()`.

PR-URL: nodejs/node#15871
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

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

@@ -25,15 +25,16 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+const fixtures = require('../common/fixtures');
+
 const assert = require('assert');
 const https = require('https');
 
 const net = require('net');
-const fs = require('fs');
 
 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'),
   handshakeTimeout: 50
 };