Add forum utility code unit test coverage

This is in preparation for significant refactoring of the code in
question.

lms/djangoapps/django_comment_client/utils.py

@@ -70,10 +70,10 @@ def get_discussion_id_map(course):
 
 def get_discussion_category_map(course):
     initialize_discussion_info(course)
-    return filter_unstarted_categories(_DISCUSSIONINFO[course.id]['category_map'])
+    return _filter_unstarted_categories(_DISCUSSIONINFO[course.id]['category_map'])
 
 
-def filter_unstarted_categories(category_map):
+def _filter_unstarted_categories(category_map):
 
     now = datetime.now(UTC())
 
@@ -111,7 +111,7 @@ def filter_unstarted_categories(category_map):
     return result_map
 
     
-def sort_map_entries(category_map, sort_alpha):
+def _sort_map_entries(category_map, sort_alpha):
     things = []
     for title, entry in category_map["entries"].items():
         if entry["sort_key"] == None and sort_alpha:
@@ -119,7 +119,7 @@ def sort_map_entries(category_map, sort_alpha):
         things.append((title, entry))
     for title, category in category_map["subcategories"].items():
         things.append((title, category))
-        sort_map_entries(category_map["subcategories"][title], sort_alpha)
+        _sort_map_entries(category_map["subcategories"][title], sort_alpha)
     category_map["children"] = [x[0] for x in sorted(things, key=lambda x: x[1]["sort_key"])]
 
 
@@ -199,7 +199,7 @@ def initialize_discussion_info(course):
                                           "sort_key": entry.get("sort_key", topic),
                                           "start_date": datetime.now(UTC())}
 
-    sort_map_entries(category_map, course.discussion_sort_alpha)
+    _sort_map_entries(category_map, course.discussion_sort_alpha)
 
     _DISCUSSIONINFO[course.id]['id_map'] = discussion_id_map
     _DISCUSSIONINFO[course.id]['category_map'] = category_map