lib: change var to let

change var to let in test-tick-processor-unknown.js
PR-URL: https://github.com/nodejs/node/pull/30408
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
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>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

test/tick-processor/test-tick-processor-unknown.js

@@ -19,7 +19,7 @@ const base = require('./tick-processor-base.js');
 base.runTest({
   pattern: /LazyCompile.*\[eval]:1|.*%  UNKNOWN/,
   code: `function f() {
-           for (var i = 0; i < 1000000; i++) {
+           for (let i = 0; i < 1000000; i++) {
              i++;
            }
            setImmediate(function() { f(); });