[FLINK-25550][k8s] Migrate flink-k8s module's test from junit4 to junit5

[reswqa]

What is the purpose of the change

Migrate flink-k8s module's test from junit4 to junit5.

Brief change log

• migrate tests from junit assert to AssertJ
• migrate tests from junit4 to junit5

Verifying this change

This change is already covered by existing tests.

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

• Dependencies : 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

[flinkbot]

CI report:

• 98c7470 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

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@flinkbot run azure

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@flinkbot run azure

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@flinkbot run azure

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@flinkbot run azure

[zentol]

I'm unable to checkout the branch. please double-check whether there isn't a trailing space in the file name:

error: invalid path 'flink-kubernetes/src/test/resources/META-INF/services/org.junit.jupiter.api.extension.Extension '

CI is also failing

[reswqa]

@zentol Thank you very much for your timely and patient reviews and suggestions. I replied to your comments for some places that I am not sure how to modify. I will submit all the modifications after I determine how to modify them here.

[reswqa]

@zentol I submitted a new commit to meet the review comment. Please have a look when you have time.