refactor: tests: boxes: Include to_write_box in WriteBox fixture.

The write_box() fixture function currently does not have a to_write_box
attribute which causes test failures for to_write_box calls inside the
keypress tests. This commit sets a default to support such calls.

Author: prah23

tests/ui_tools/test_boxes.py

@@ -29,6 +29,8 @@ def write_box(self, mocker, users_fixture, user_groups_fixture,
             {'name': stream['name']} for stream in
             streams_fixture], key=lambda stream: stream['name'].lower())
 
+        write_box.to_write_box = None
+
         return write_box
 
     def test_init(self, write_box):