Coverage for Assign and Unassign of Regions on RegionServer on failure

Looking at 'HBASE-23984 [Flakey Tests] TestMasterAbortAndRSGotKilled fails in teardown', and at UnassignRegionHandler, AssignRegionHandler, CloseRegionHandler, and the work that is done inline w/ request vs that which to the side in executors, we need more coverage and specification of what happens around the edges. This coverage would be more to see if holes in our handling currently in a unit test case context before we see it out on clusters.

HBASE-23984 addresses holes where UnassignRegionHandler and AssignRegionHandler could skip out w/o clearing Regions from the RegionServer#regionsInTransitionInRS Map of Regions In Transition if failed open or close because the RegionServer is aborting.

Other holes seem lurking. On exception, we were leaving entries in the RegionServer# submittedRegionProcedure map added by HBASE-2204; not the end of the world but they should be cleared on error? HBASE-23984 adds clearning from submittedRegionProcedure but then procedures even if failed get added to the cache of procedures... so if we try to run the procedure again against this server it won't be scheduled.

interesting stuff.

This issue is about adding tests that fail assign/unassign/close on the RegionServer side making sure RS state is left in a good condition on fail.