Update unstable lint docs to include required feature attributes
closes#135298
## Summary
This PR updates the documentation examples for the following unstable lints to ensure they include the necessary feature attributes for proper usage:
- fuzzy_provenance_casts
- lossy_provenance_casts
- unqualified_local_imports
- test_unstable_lint
## Changes Made:
- Added the appropriate #![feature(...)] attributes to the example code for each lint.
- Updated the examples to produce correct and meaningful warnings, ensuring they align with current lint behavior.
Reference:
- Used the `must_not_suspend` lint documentation as a template for these updates.
Guillaume Gomez
d8e88c9751