Teach Cocoon test bot about pending changelogs #191744

flutter/packages#12620 is an example of a PR where normally the bot would have recognized that the change was comment-only, and didn't need an exemption, but the new batch release pending changelog system caused it to flag it for manual exemption.

https://github.com/flutter/cocoon/blob/f469832acc0e13f3ef06236b315d8421c6be8455/app_dart/lib/src/service/pull_request_manager.dart#L710-L717 should be updated to ignore things in pending_changelogs directories for the packages repo.