Core: DiskThresholdDecider#remain(...) should take shards relocating away into account

Let the disk threshold decider take into account shards moving away from a node in order to determine if a shard can remain.

By taking this into account we can prevent that we move too many shards away than is necessary.

PR for #8538
You put `[test][0][p]` and `[test][1][p]`, but then create metadata with 1 primary shard and one replica, can you change it here or in the shardSizes map?