[FLINK-27942][tests] Migrate flink-connectors-rabbitmq to JUnit5

[snuyanzin]

What is the purpose of the change

1. Migrate flink-connectors-rabbitmq module to AssertJ and JUnit 5 following the JUnit 5 Migration Guide
2. Fix RestClusterClient injection

Brief change log

Use JUnit5 and AssertJ in tests instead of JUnit4 and Hamcrest

Verifying this change

This change is a 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 applicable)

[flinkbot]

CI report:

• 74ddcaf UNKNOWN
• cfad314 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build

[snuyanzin]

@flinkbot run azure

[XComp]

LGTM 👍 Thanks for creating the PR, @snuyanzin . I have a few minor cosmetic comments.

[XComp]

LGTM 👍 Please squash the commits and rebase the branch.

[snuyanzin]

@flinkbot run azure

[snuyanzin]

Thanks for the feedback

Please squash the commits and rebase the branch.

done