Add test opening 2500 cores

[mjtognetti]

Adding this to test hitting the max open file limit. It surfaces some limitations with the current implementation of repo.close(), noted in the comments.

Repeated here: This timeout is a hack - we don't have a way to safely close the repo. In this case, we have a feed.ready() callback which writes to the FeedInfoStore db. This callback throws if the database has been closed. We need a way to either safely shut down (build a queue and let it drain, making repo.close() async) - or - we should implement a hard shut down which clears those queues.