[FLINK-16690][task][tests] Refactor StreamTaskTest to reuse TestTaskBuilder and MockStreamTaskBuilder

[zhijiangW]

What is the purpose of the change

We can reuse existing TestTaskBuilder and MockStreamTaskBuilder for constructing Task and StreamTask easily in tests to simplify StreamTaskTest case.

Brief change log

see commit messages

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): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

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Automated Checks

Last check on commit b9006b6 (Fri Mar 20 07:25:57 UTC 2020)

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CI report:

• 9da7f28 UNKNOWN
• fe213e9 Travis: SUCCESS Azure: FAILURE

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[pnowojski]

Nice changes :) LGTM % with green travis & azure