Today closing a `ClusterNode` in an `AbstractCoordinatorTestCase` uses
`onNode()` so has no effect if the node is not in the current list of nodes.
It also discards the `Runnable` it creates without having run it, so has no
effect anyway.
This commit makes these tests much stricter about properly closing the nodes
started during `Coordinator` tests, by tracking the persisted states that are
opened, and adds an assertion to catch the trappy requirement that the closing
node still belongs to the cluster.
55f1dd8da6