Test _template_folders functions via TemplateList

This is part of the overall migration of the "_template_folders"
methods to the TemplateList class. Moving the existing tests now
will make the actual migration easier to follow.

To emulate the second and third tests, we need to grab a specific
folder from the TemplateList and then look at its folders - these
are set based on "get_template_folders" as before.

app/models/template_list.py

@@ -170,6 +170,7 @@ class TemplateListFolder(TemplateListItem):
         super().__init__(folder, ancestors)
         self.folder = folder
         self.service_id = service_id
+        self.folders = folders
         self.number_of_templates = len(templates)
         self.number_of_folders = len(folders)