test: replaces fixturesDir with fixtures

This commit updates the common.fixturesDir method
in the https-timeout test.

PR-URL: #15835
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-https-timeout.js

@@ -25,13 +25,12 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+const fixtures = require('../common/fixtures');
 const https = require('https');
 
-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')
 };
 
 // a server that never replies