Improve subscription test to verify enabled status
Improve subscription test to verify enabled status

Check that we get a valid state back from the callback, and verify that
the enabled flag has been updated to reflect our prior call to
riak_ensemble_manager:enable().

ricardobcl/riak_ensemble,sargun/riak_ensemble,basho/riak_ensemble,ricardobcl/riak_ensemble,sargun/riak_ensemble,basho/riak_ensemble