test: change var to const in ./common

PR-URL: https://github.com/nodejs/node/pull/13732
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

test/common/index.js

@@ -428,8 +428,7 @@ exports.allowGlobals = allowGlobals;
 function leakedGlobals() {
   const leaked = [];
 
-  // eslint-disable-next-line no-var
-  for (var val in global) {
+  for (const val in global) {
     if (!knownGlobals.includes(global[val])) {
       leaked.push(val);
     }