Consolidate completion test modules

What does this merge request do and why?

• Relocate tests in tests/promptlib/completions to tests/promptlib/completion to mirror the implementation structure.
• Move each model tests in test_model_handles.py to separate files.
• Add some rename to taste

How to set up and validate locally

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

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.