Test infrastructure: improve test organization and documentation

[robtaylor]

Summary

The test infrastructure could benefit from better organization and documentation.

Current Issues

Test organization

• Test files spread across multiple locations
• Inconsistent naming conventions
• No clear categorization of what each test covers
• Some tests in source files, some in test_data

Documentation

• No test README explaining the test structure
• No documentation of what each test category covers
• No guide for adding new tests (partially addressed in PR Add developer documentation with testing guide #41)

Test tooling

• mini_harness custom framework lacks documentation
• No test coverage reporting
• No way to easily run subset of tests by category

CI concerns

• Some tests skipped on Windows (if stdx::IS_CI && cfg!(windows))
• Integration tests disabled by default (RUN_DEV_TESTS)
• Comment mentions tests are "flakey" regarding linker invocation

Suggested Improvements

1. Add test README - Document test structure and how to add tests
2. Categorize tests - Group by feature area (lexer, parser, HIR, MIR, etc.)
3. Add coverage reporting - Use cargo-llvm-cov or similar
4. Fix flakey tests - Investigate and fix CI reliability issues
5. Enable more tests in CI - Consider enabling integration tests
6. Add test templates - Provide example tests for common patterns

Related

• PR Add developer documentation with testing guide #41 adds some documentation about testing
• Issue Clean up commented-out code in xtask #47 about cleaning up commented-out code