[FLINK-27455][tests] Migrate SnapshotMigrationTestBase to JUnit5

[Jiabao-Sun]

What is the purpose of the change

[FLINK-27455][tests] Migrate SnapshotMigrationTestBase to JUnit5

Brief change log

Migrate SnapshotMigrationTestBase to JUnit5

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not documented)

[flinkbot]

CI report:

• 5fd3aeb Azure: SUCCESS

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[Jiabao-Sun]

Hi @zentol, could you help review it when you have time?