Clarify backend test modes and document inject/expect in ddiff integration tests

[junwha]

Motivation

• Make the ddiff backend integration suite easier to read and extend by documenting the common lifecycle and the per-mode intent.
• Surface what each mode injects and what it should verify so new modes can be added without interleaving concerns.

Description

• Replace the previous monolithic test script with a documented test/test_ddiff_backends.sh that centralizes helpers (run_step, wait_for_registry, have_all_commands, cleanup_case) and the shared run_case workflow.
• Introduce explicit, dedicated mode functions run_docker_mode, run_docker_skopeo_mode, and run_podman_mode with Inject and Expect comments describing environment flags and log markers.
• Add lightweight test images under test/dockerfiles as Dockerfile.base and Dockerfile.delta to provide the base/delta images used by the flow.
• Keep behavior: build base/delta, run ddiff diff, recreate registry, run ddiff load, verify loaded image contains tmux, and clean up logs/artifacts, while emitting clear PASS/FAIL/SKIP accounting.

Testing

• Ran bash -n test/test_ddiff_backends.sh which completed with no syntax errors.
• Executed ./test/test_ddiff_backends.sh podman in the current environment, which correctly reported missing prerequisites (podman and skopeo) and produced a SKIP result with totals Total: 1, Passed: 0, Failed: 0, Skipped: 1.

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Codex Task