[FLINK-36217][tests] Remove powermock usage

[snuyanzin]

What is the purpose of the change

Remove powermock from usages

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 applicable)

[flinkbot]

CI report:

• e2e9a73 Azure: SUCCESS

Bot commands

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

[snuyanzin]

@GOODBOY008 I noticed you made a number of changes in these files
may I ask you to have a look?

[GOODBOY008]

@snuyanzin Thanks for your contributions. I left some comment.