Minor Description
Right now, tests based on InternalMiniCluster use direct access to Master and TServer interfaces to invoke master/tserver calls. The tests based on ExternalMiniCluster use mix of ExternalMaster, ExternalTabletServer/TServerDetails wrappers to invoke master/tserver RPC methods. Two different wrappers exist to do the same things in Internal and External mini clusters, and in many cases the tests which use InternalMiniCluster could invoke necessary methods via RPC as well (instead of direct in-process calls) and use common test wrapper library.
Also, from the pure API point, it would be much cleaner to have InternalMiniCluster and ExternalMiniCluster provide common interface to the common functionality they have (in this particular case, to invoke RPC methods of masters/tservers).