test: generalize integration tests

What does this merge request do and why?

This MR generalizes the integration tests, which was introduced in chore: Add e2e test for CS user auth flow (!919 - merged). Thanks to @alipniagov .

I think we should expand these integration test cases, at least cover happy path for each endpoint. To do so, we need to generalize self-signed JWT endpoint.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.