App tests

[lebaaar]

There are currently absolutely zero tests written. Add tests to test/.
Tests should cover all the core app functionality:

• Adding period
• Editing period
• Predicts correct next period date
• Show correct values on insights page
• Check for scheduled notifications (helper method is in /lib/services/notification_service.dart - getPendingNotifications)
• Transfer data
• Restore data
• ...

Also cover negative tests!