test: use fixtures module

Replace common.fixturesDir with fixtures module usage in
test/parallel/test-tls-fast-writing.js.

PR-URL: nodejs/node#15808
Reviewed-By: Ryan Graham <r.m.graham@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-tls-fast-writing.js

@@ -24,14 +24,13 @@ const common = require('../common');
 if (!common.hasCrypto)
   common.skip('missing crypto');
 
+const fixtures = require('../common/fixtures');
 const assert = require('assert');
 const tls = require('tls');
-const fs = require('fs');
 
-const dir = common.fixturesDir;
-const options = { key: fs.readFileSync(`${dir}/test_key.pem`),
-                  cert: fs.readFileSync(`${dir}/test_cert.pem`),
-                  ca: [ fs.readFileSync(`${dir}/test_ca.pem`) ] };
+const options = { key: fixtures.readSync('test_key.pem'),
+                  cert: fixtures.readSync('test_cert.pem'),
+                  ca: [ fixtures.readSync('test_ca.pem') ] };
 
 const server = tls.createServer(options, onconnection);
 let gotChunk = false;