test: replace string concatenation with template

PR-URL: #16916
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Daniel Bevenius <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Franziska Hinkelmann <[email protected]>

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}`);
   });
 }