test: replace string concatenation with template

PR-URL: #16916
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/fixtures/cluster-preload-test.js

@@ -2,6 +2,6 @@ var cluster = require('cluster');
 if (cluster.isMaster) {
   cluster.fork(); // one child
   cluster.on('exit', function(worker, code, signal) {
-    console.log('worker terminated with code ' + code);
+    console.log(`worker terminated with code ${code}`);
   });
 }