test: add ProxySQL integration test suite

27 tests covering:
- Provider registry (mysql + proxysql listed)
- Standalone ProxySQL deploy/connect/stop
- Single MySQL + ProxySQL (1 backend in HG 0)
- Replication + ProxySQL (master in HG 0, slaves in HG 1)
- Data flow through ProxySQL to MySQL with replication verification
- ProxySQL health monitoring (detect stopped slave)
- Concurrent connections (20 parallel queries)
- Runtime admin modifications via admin interface
- Multiple independent sandboxes coexistence
- Configuration correctness (datadir, credentials, monitor user, scripts)
- Idempotency (deploy, delete, redeploy)

Known issue: test isolation requires clean port state between sections.
Run with: ./test/proxysql-integration-tests.sh [sandbox-binary-dir]

Author: renecannao