Test if multiple prefixes for the same IRI persist in a namespace manager

[white-gecko]

Summary of changes

This pull request so far introduces some failing tests, that demonstrate that the namespace manager is not able to maintain multiple prefixes for the same IRI, as mentioned in #2077 .

Roughly related to #2779, as it also cares about parsing/serialization side effects.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes. 🛑
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement.
  • Considered adding an example in ./examples.
• If the change has a potential impact on users of this project:
  • Added or updated tests that fail without the change.
  • Updated relevant documentation to avoid inaccuracies.
  • Considered adding additional documentation.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[nicholascar]

@white-gecko are you going to move this from Draft to ready?

[white-gecko]

So far these are just failing tests, for which it is required to fix the namespace manager accordingly. So it is not ready to be merged until an implementation follows. You could also close it, to have it out of the way and we reopen it, once an improved namespace manager is available.