Add MIROIR_TEST_SKIP_DOCKER and MIROIR_TEST_REDIS_URL environment variables
to allow Redis integration tests to run without Docker or use external Redis.
Changes:
- Modified setup_redis_store() to support external Redis via MIROIR_TEST_REDIS_URL
- Added skip_if_no_redis!() macro for graceful test skipping
- Tests now skip with clear message when Docker unavailable
- Added docs/TESTING.md with test environment documentation
Fixes bead bf-5qy60: Fix Redis integration tests infrastructure
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
