test: use fixtures in test-https-set-timeout-server

Task to replace the common.fixturesDir with the usage
of the common.fixtures module. At Node.js Interactive.
First PR to Node.js. Yay!

PR-URL: nodejs/node#15886
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

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

@@ -26,16 +26,16 @@ if (!common.hasCrypto)
   common.skip('missing crypto');
 
 const assert = require('assert');
-const fs = require('fs');
+const fixtures = require('../common/fixtures');
 const https = require('https');
 const http = require('http');
 const tls = require('tls');
 
 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) {