Ensure GridFS spec tests fully iterate expected and actual collections

The GridFS spec test runner uses a MultipleIterator to compare expected and actual collections, but was not using the MIT_NEED_ANY flag, which lead to comparisons stopping when the first cursor was exhausted.

Changing to MIT_NEED_ANY will also require we remove orphan chunks leftover from deleting a nonexistent file from the expected collection, since that is optional behavior and the spec tests intentionally do not assert that they are missing (while PHPLIB should do so).