Remove redundant test documentation files (refactor summary and coverage report)

These files were intermediate artifacts from the test refactoring work and should not be permanent documentation:
- tests/README-refactor.md - Historical refactoring summary
- tests/COVERAGE_REPORT.md - Point-in-time coverage snapshot

The active test documentation remains in tests/README.md and tests/README-zh-CN.md.

Co-authored-by: wysaid <1430725+wysaid@users.noreply.github.com>

Author: Copilot