docs: remove redundant README_TESTS.md from Catalogs module

The README_TESTS.md file contained module-specific test documentation that:
- Duplicated information already centralized in docs/testing/
- Was outdated (didn't reflect recent refactorings)
- Was not a pattern followed by other modules (Users, Providers, etc.)

All test documentation is now properly centralized in:
- docs/testing/integration_tests.md
- docs/testing/test_infrastructure.md
- docs/testing/code_coverage_guide.md
- docs/testing/test_auth_examples.md

This improves maintainability by avoiding documentation drift between
module-specific files and centralized docs.