testsuite: Allow accessors in interfaces

testsuite: Allow accessors in interfaces

There are a lot of cases where properties are implemented in classes but
the getters for these exist in an interface that class implements.

A common Example is g_list_model_get_n_items() being the getter for
GtkWhateverListModel::n-items.
But also property implementations that don't use override_property()
(usually because they have a different default) are handled by this.