test: Change from var to const

PR-URL: #30431
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

test/parallel/test-vm-module-link.js

@@ -97,11 +97,11 @@ async function circular2() {
     `,
     './a.mjs': `
       export * from './b.mjs';
-      export var fromA;
+      export let fromA;
     `,
     './b.mjs': `
       export * from './a.mjs';
-      export var fromB;
+      export let fromB;
     `
   };
   const moduleMap = new Map();