test: add required flag to FinalizationGroup test

`cleanupSome` was moved to a separate flag due to discussions in the
last TC39 meeting. Use this flag to ensure the test passes.

Refs: https://chromium-review.googlesource.com/c/v8/v8/+/2141011
Fixes: #155

Author: mmarchini

test/parallel/test-finalization-group.js

@@ -1,6 +1,6 @@
 'use strict';
 
-// Flags: --expose-gc --harmony-weak-refs
+// Flags: --expose-gc --harmony-weak-refs --harmony-weak-refs-with-cleanup-some
 
 const common = require('../common');