Add tests for user_group_distinguished_names()

In https://github.com/stackabletech/opa-operator/pull/622#top we added an Active Directory backend to the user info fetcher.

As part of this, there was fairly extensive code to convert between some weird representations which AD chose for userids and groupids.

In order to not block the release with this we decided to punt on adding unit tests (hopefully, worst case: integration tests) for this later.

We should try and get this done fairly soon after 24.11.

Issue checklist

This is a simple checklist of things to bear in mind when creating a new issue.

• Describe the use-case, as far is possible. For instance, using the pattern "As a XXXX, I would like XXXX to be able to do XXXX" helps to identify the feature as well as the problem it is intended to address.
• Indicate an approximate level of importance and urgency.
• Indicate if there is a known work-around until such time as the issue has been implemented.

Start with the implementation and conversion code added in pull request 622, especially the user_group_distinguished_names() path. Add unit tests, or integration tests if necessary, covering the Active Directory user and group ID representations; done means the conversion behavior is exercised without blocking the release.