Generate tests for the user service

[tschaffter]

Automatically generate tests for the user service with the OpenAPI generator and template if needed.

See:

- OpenAPI Tutorial: How to Automatically Generate Tests for OpenAPI Generator SDKs

Related tasks:

• Setup unit and integration tests for the user service #481
• Run user service integration tests with Gradle #569

[tschaffter]

This would be nice to have by the time the user service is completed still this sprint (22.11). The knowledge of how to add unit and integration tests for micro-services would greatly improve the robustness of the code base. Most of the tests could be copy-pasted-adapted from one service to another.