add MariaDB source smoke test and next-gen integration test support

[joechenrh]

Background

DM integration tests currently only run against classic TiDB (unistore mode). To validate DM compatibility with next-gen TiDB (Cloud Storage Engine edition), we need to:

1. Add infrastructure to start a full next-gen cluster (MinIO + PD + TiKV + tikv-worker + SYSTEM TiDB + user TiDB)
2. Adapt existing tests for next-gen compatibility differences
3. Add a MariaDB source smoke test to cover MariaDB upstream replication

What needs to be done

• Add mariadb_source integration test case
• Add next-gen cluster startup scripts (run_downstream_cluster_nextgen, run_downstream_cluster_with_tls_nextgen)
• Centralize cluster lifecycle in cluster_lib.sh
• Adapt test scripts for next-gen (import-into mode, session normalization, privilege changes, collation defaults, etc.)
• Fix pre-existing flakes (cleanup_process etcd quorum hang, print_status infinite loop)
• Validate all 13 groups (G00–G11 + TLS_GROUP) pass on both classic and next-gen CI

[joechenrh]

/type enhancement