Improve test coverage

Problem/Motivation

To ensure code quality, unit tests and other relevant tests should be added.

Steps to reproduce

• Check the source code for relevant test files

Proposed resolution

Add relevant unit, kernel and browser tests to enhance test coverage.

Remaining tasks

• Write tests for the various API endpoints the module provides, e.g.
• Subscribe endpoint /api/v1/de_notifications/subscribe
• Confirm endpoint /api/v1/de_notifications/confirm
• Unsubscribe endpoint /api/v1/de_notifications/unsubscribe
• Unsubscribe All endpoint api/v1/de_notifications/unsubscribe/all
• Request subscription overview endpoint /api/v1/de_notifications/request_subscription_overview
• Add tests to test specific functionality of the module, e.g. NotificationsTokenService or NotifySubscribers

User interface changes

• None

API changes

• None

Data model changes

• None