Write more tests

[rafaelurben]

Examples:

• Check all admin URLs while authenticated (500 check for template errors or admin config mistakes)
• Check all custom URLs
• Check all custom URLs while not authenticated (expect redirect)
• Check order total calculations
• Check product sale price calculations
• Check Swiss QR payload generation
• Check PDF generation
• Check if makemigrations wants to create new migrations
• Check emails?

Other tasks:

• Rename tests directory (and all references) to kmuhelper_tests
• Add django system check (https://docs.djangoproject.com/en/5.1/ref/django-admin/#check)
• Add manage.py test kmuhelper_tests to private django app (and add test database to databases)

[rafaelurben]

Note: KMUHelper will not be tested thoroughly. Tests are mainly there to prevent common mistakes leading to internal server errors and detecting breaking changes in dependencies.