View theme override in drupal 8
In your case 'views_view_unformatted__test_content_view' is the theme hook in the render array. In Drupal 8 that will make it possible to use following preprocess functions to modify the variables before they get to the template:


themename_preprocess_views_view_unformatted__test_content_view()
themename_preprocess_views_view_unformatted()


There is more information about preprocess functions available here.