Use hashtables for plugin disco features

tests/unittests/test_plugins_disco.h

@@ -0,0 +1,7 @@
+void returns_empty_list_when_none(void **state);
+void returns_added_feature(void **state);
+void resets_features_on_close(void **state);
+void returns_all_added_features(void **state);
+void does_not_add_duplicate_feature(void **state);
+void removes_plugin_features(void **state);
+void does_not_remove_feature_when_more_than_one_reference(void **state);