test: change concatenated string to template

PR-URL: #16912
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/fixtures/loop.js

@@ -4,7 +4,7 @@ console.log('A message', 5);
 while (t > 0) {
   if (t++ === 1000) {
     t = 0;
-    console.log('Outputed message #' + k++);
+    console.log(`Outputed message #${k++}`);
   }
 }
 process.exit(55);