[FLINK-27937][tests][pubsub] Migrate flink-connectors-gcp-pubsub to JUnit 5

[snuyanzin]

What is the purpose of the change

Migrate flink-connectors-gcp-pubsub module to AssertJ and JUnit 5 following the JUnit 5 Migration Guide

Brief change log

1. Use JUnit5 and AssertJ in tests instead of JUnit4 and Hamcrest
2. Add dependency on mockito-junit-jupiter since pubsub connector requires it

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): (yes)
• 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:

• 37e4f95 Azure: SUCCESS

Bot commands

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

[snuyanzin]

@flinkbot run azure

[XComp]

Thanks for your contribution @snuyanzin. In general, the PR looks good. I just have one comment where I need a bit of clarification.