test: replace string concatenation with template

PR-URL: nodejs#14286
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

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

@@ -9,8 +9,8 @@ common.refreshTmpDir();
 const fs = require('fs');
 const https = require('https');
 const options = {
-  cert: fs.readFileSync(common.fixturesDir + '/test_cert.pem'),
-  key: fs.readFileSync(common.fixturesDir + '/test_key.pem')
+  cert: fs.readFileSync(`${common.fixturesDir}/test_cert.pem`),
+  key: fs.readFileSync(`${common.fixturesDir}/test_key.pem`)
 };
 
 const server = https.createServer(options, common.mustCall((req, res) => {