Tighten assertions in MoveDecision static method helpers

[DiannaHohensee]

I suspect assertions could be added to MoveDecision#rebalance but I haven't taken a look at that code so far.

[elasticsearchmachine]

Pinging @elastic/es-distributed-coordination (Team:Distributed Coordination)

[DiannaHohensee]

Aha, I wasn't expecting THROTTLE in canRemain. Interesting 🤔

  2> java.lang.AssertionError: Create decision with MoveDecision#move instead. canRemain Decision: THROTTLE()
        at __randomizedtesting.SeedInfo.seed([5A18EE3BD8858B6A:229FE7E887BE5D23]:0)
        at org.elasticsearch.cluster.routing.allocation.MoveDecision.createRemainYesDecision(MoveDecision.java:97)
        at org.elasticsearch.cluster.routing.allocation.allocator.BalancedShardsAllocator$Balancer.decideMove(BalancedShardsAllocator.java:930)
        at org.elasticsearch.cluster.routing.allocation.allocator.BalancedShardsAllocator$Balancer.moveShards(BalancedShardsAllocator.java:819)
        at org.elasticsearch.cluster.routing.allocation.allocator.BalancedShardsAllocator.allocate(BalancedShardsAllocator.java:183)
        at org.elasticsearch.cluster.routing.allocation.allocator.ShardsAllocator.allocate(ShardsAllocator.java:50)
        at org.elasticsearch.cluster.routing.allocation.AllocationService.lambda$reroute$7(AllocationService.java:453)
        at org.elasticsearch.cluster.routing.allocation.AllocationService.reroute(AllocationService.java:590)
        at org.elasticsearch.cluster.routing.allocation.AllocationService.executeWithRoutingAllocation(AllocationService.java:469)
        at org.elasticsearch.cluster.routing.allocation.AllocationService.reroute(AllocationService.java:450)
        at org.elasticsearch.cluster.metadata.MetadataDeleteIndexService$1.afterBatchExecution(MetadataDeleteIndexService.java:73)
        at org.elasticsearch.cluster.SimpleBatchedAckListenerTaskExecutor.execute(SimpleBatchedAckListenerTaskExecutor.java:72)
        at org.elasticsearch.cluster.service.MasterService.innerExecuteTasks(MasterService.java:1100)
        at org.elasticsearch.cluster.service.MasterService.executeTasks(MasterService.java:1063)
        at org.elasticsearch.cluster.service.MasterService.executeAndPublishBatch(MasterService.java:246)
        at org.elasticsearch.cluster.service.MasterService$BatchingTaskQueue$Processor.lambda$run$2(MasterService.java:1710)
        at org.elasticsearch.action.ActionListener.run(ActionListener.java:465)
        at org.elasticsearch.cluster.service.MasterService$BatchingTaskQueue$Processor.run(MasterService.java:1707)
        at org.elasticsearch.cluster.service.MasterService$5.lambda$doRun$0(MasterService.java:1308)
        at org.elasticsearch.action.ActionListener.run(ActionListener.java:465)
        at org.elasticsearch.cluster.service.MasterService$5.doRun(MasterService.java:1287)
        at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:27)
        at org.elasticsearch.common.util.concurrent.EsExecutors$DirectExecutorService.execute(EsExecutors.java:315)
        at org.elasticsearch.common.util.concurrent.StoppableExecutorServiceWrapper.execute(StoppableExecutorServiceWrapper.java:93)
        at org.elasticsearch.cluster.service.MasterService.forkQueueProcessor(MasterService.java:1371)
        at org.elasticsearch.cluster.service.MasterService$PerPriorityQueue.execute(MasterService.java:1418)
        at org.elasticsearch.cluster.service.MasterService$BatchingTaskQueue.submitTask(MasterService.java:1630)
        at org.elasticsearch.cluster.metadata.MetadataDeleteIndexService.deleteIndices(MetadataDeleteIndexService.java:94)
        at org.elasticsearch.action.admin.indices.delete.TransportDeleteIndexAction.masterOperation(TransportDeleteIndexAction.java:106)
        at org.elasticsearch.action.admin.indices.delete.TransportDeleteIndexAction.masterOperation(TransportDeleteIndexAction.java:41)
        at org.elasticsearch.action.support.master.TransportMasterNodeActionUtils.runMasterOperation(TransportMasterNodeActionUtils.java:31)
        at org.elasticsearch.indices.cluster.ClusterStateChanges.execute(ClusterStateChanges.java:546)
        at org.elasticsearch.indices.cluster.ClusterStateChanges.deleteIndices(ClusterStateChanges.java:403)
        at org.elasticsearch.indices.cluster.IndicesClusterStateServiceRandomUpdatesTests.randomlyUpdateClusterState(IndicesClusterStateServiceRandomUpdatesTests.java:403)
        at org.elasticsearch.indices.cluster.IndicesClusterStateServiceRandomUpdatesTests.testRandomClusterStateUpdates(IndicesClusterStateServiceRandomUpdatesTests.java:111)