enhance decommission test to check that node was properly decommissioned

Decommissioning a node has two parts - the process of CockroachDB removing all data from that node, and then the process of removing that node from the SS so it doesn't get restarted after it's shut down. Our current test only checks the latter. To check the former, we need to check `cockroach node list` and make sure that the node is listed in a decommissioned state and not just as failed. cc: @udnay and @chrislovecnm