Write functional tests for the controllers

[oliverklee]

Goal

We’d like to have functional tests for the controllers so we know they work correctly without relying on the implementation details too much. (We have a consensus in the team on this.)

Acceptance Criteria

• [ ] We have all relevant functionality of the controllers covered in functional tests.
• [ ] This happens in small steps, i.e., one PR per controller.
• [ ] All functionality (not the structural properties) that currently is covered by the unit tests is covered by the new functional tests.

[oliverklee]

@andreaswolf As you had brought up that you find functional tests for controllers important, would you be willing to step up and do the work of creating the PRs? This would help speed things up tremendously.