Add regression test for memory leak during planning

Recently we discovered a memory leak during planning where a partial plan is built for a subtree of the query, then we later find that we cannot index another branch of the query and discard the larger plan in a way that leaks the partial plan.

This is known to be fixed by SERVER-35455 so this is simply adding a regression test that we can use to verify the fix on older branches while backporting SERVER-35455.