test: add clear-search URL and template tests

Add tests for clear_search_url context and template clear-search button when a search is active.

viewer/directory.py

@@ -169,6 +169,9 @@ def render_directory(request, path_text, full_path):
         "search_action_url": gallery_url(
             Path(path_text) if path_text != "" else None, True, search_action_query
         ),
+        "clear_search_url":  gallery_url(
+            Path(path_text) if path_text != "" else None, True, None
+        ),
     }
 
     # sort_label depends on SORT_LABELS in common; import lazily to avoid circulars