test: use fixtures.readSync

PR-URL: #15841
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: szhang351

test/parallel/test-https-drain.js

@@ -3,14 +3,13 @@ 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 fs = require('fs');
-const path = require('path');
 
 const options = {
-  key: fs.readFileSync(path.join(common.fixturesDir, 'test_key.pem')),
-  cert: fs.readFileSync(path.join(common.fixturesDir, 'test_cert.pem'))
+  key: fixtures.readSync('test_key.pem'),
+  cert: fixtures.readSync('test_cert.pem')
 };
 
 const bufSize = 1024 * 1024;