test: code&learn var to let update

PR-URL: #30436
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

Author: Nazaroni

test/parallel/test-repl-save-load.js

@@ -44,8 +44,8 @@ testMe._domain.on('error', function(reason) {
 });
 
 const testFile = [
-  'var top = function() {',
-  'var inner = {one:1};'
+  'let top = function() {',
+  'let inner = {one:1};'
 ];
 const saveFileName = join(tmpdir.path, 'test.save.js');