Add tests to Pomodoro plugin

Features to test:
- `set` and `unset` commands with help
- `PomodoroPlugin.alarm(bot, job)`
- `PomodoroPlugin.set_timer(bot, update, args, job_queue, chat_data)`
- `PomodoroPlugin.unset_timer(bot, update, chat_data)`

Fakes/spies to implement:
- `job`
- `bot`
- `update`
- `job_queue`