Add integration testsuite

[SchoolGuy]

Is your feature request related to a problem?

As a user of the CLI
I want to not be the beta tester of the application
so that I can be productive using the new CLI.

Provide a detailed description of the proposed feature

We need to ensure code quality. As such we need to ensure that the CLI runs smoothly with the different Cobbler server versions. This means we need an integration testsuite that can test multiple versions of the backend at the same time or after another.

Since the user experience is largely dependent on the experience in error cases we also need to ensure that error handling is adequate. If doing so is not possible in the GitHub CI then documentation on how to verify this manually is required.

Alternatives you've considered

None

Additional information

None