test: use template literals in test-writewrap

Use template literals instead of string concatenation in
test/async-hooks/test-writewrap.js.

PR-URL: #14292
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

test/async-hooks/test-writewrap.js

@@ -18,8 +18,8 @@ hooks.enable();
 //
 const server = tls
   .createServer({
-    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`)
   })
   .on('listening', common.mustCall(onlistening))
   .on('secureConnection', common.mustCall(onsecureConnection))