Extract host-test harness to mk/testing/host.mk

[pbalduino]

Parent Issue

Part of #352 - Makefile refactoring

Objective

Move the long host-test harness so CI/test tweaks don't require scrolling through build logic.

Scope

Extract from root Makefile:

• Lines ~700: Host-test harness
• Test target definitions
• Test environment setup
• CI-related test configuration

Target Location

mk/testing/host.mk

Implementation Steps

1. Create mk/testing/ directory structure
2. Extract test harness and related rules
3. Move test environment configuration
4. Update root Makefile to include mk/testing/host.mk
5. Run make test or equivalent
6. Verify all tests pass
7. Test in CI environment if applicable

Dependencies

None - testing can be isolated

Acceptance Criteria

• [ ] All test harness code in mk/testing/host.mk
• [ ] make test works correctly
• [ ] All tests pass as before
• [ ] CI builds work
• [ ] Test configuration is easy to find and modify
• [ ] No behavioral changes