test: change concatenated string to template

PR-URL: #16912
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Franziska Hinkelmann <[email protected]>

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);