Tests for HBASE-18408 "AM consumes CPU and fills up the logs really fast when there is no RS to
assign". If an UnexpectedStateException, we'd spin on
the ProcedureExecutor consuming CPU and filling logs. Test new back-off facility.
Manufacture a state that will throw UnexpectedStateException.
Change an assigned region's 'state' to be OPENING. That'll mess up a subsequent unassign
causing it to throw UnexpectedStateException. We can easily manufacture this infinite retry
state in UnassignProcedure because it has no startTransition. AssignProcedure does where it
squashes whatever the current region state is making it OFFLINE. That makes it harder to mess
it up. Make do with UnassignProcedure for now.