test: fix permanent deoptimizations

PR-URL: #12456
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: mscdex

test/common.js

@@ -385,7 +385,8 @@ exports.allowGlobals = allowGlobals;
 function leakedGlobals() {
   const leaked = [];
 
-  for (const val in global)
+  // eslint-disable-next-line no-var
+  for (var val in global)
     if (!knownGlobals.includes(global[val]))
       leaked.push(val);