As of SERVER-126201 we now perform cleanup of stale collection entries that have no chunks visible on the shard.

However, this cleanup only occurs when the oldest timestamp visible on the shard advances past the stale chunk's last modification timestamp (onCurrentShardSince). Our current tests do not stress this scenario of having short timestamp windows with concurrent chunk balancing.

This ticket is about adding some form of testing such that the sharding catalog cleanup process can be tested in the presence of concurrent operation.