feat(todos): Support filtering by group_id.

This MR does two things:

• Add the ability for TodosService.ListTodos to filter by group_id.

  API docs: https://docs.gitlab.com/api/todos/#get-a-list-of-to-do-items

• Improve the testing of TodosService.ListTodos

To facilitate the testing, the following test helpers have been added:

• testParam – test the value of a single URL parameter
• mustWriteJSONResponse – write an arbitrary type to the response in JSON format
• mustWriteErrorResponse – format an error so it can be parsed by parseError()