test: use fixtures module

PR-URL: https://github.com/nodejs/node/pull/16034
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-https-unix-socket-self-signed.js

@@ -6,11 +6,11 @@ if (!common.hasCrypto)
 
 common.refreshTmpDir();
 
-const fs = require('fs');
+const fixtures = require('../common/fixtures');
 const https = require('https');
 const options = {
-  cert: fs.readFileSync(`${common.fixturesDir}/test_cert.pem`),
-  key: fs.readFileSync(`${common.fixturesDir}/test_key.pem`)
+  cert: fixtures.readSync('test_cert.pem'),
+  key: fixtures.readSync('test_key.pem')
 };
 
 const server = https.createServer(options, common.mustCall((req, res) => {