fix #929: wait for riak_kv_stat to unregister

The {{already_started, ...}, {child, ...}} error noted in the description
of issue #929 is caused by an instance of riak_kv_stat trying to register
its name when another instance is already registered. Add
riak_kv_test_util:wait_for_unregister/1 and use it in
riak_kv_test_util:cleanup/3 to wait for the registered instance of
riak_kv_stat to unregister and stop. Remove similar functionality in
test/get_put_monitor_eqc.erl and make that module call
riak_kv_test_util:wait_for_unregister/1 instead.

Also add goldrush to the app list in riak_kv_test_util:dep_apps/2 to ensure
it's started and stopped correctly; this prevents crash dumps caused by
{application_terminated,goldrush,killed} at the end of "make test".