[FLINK-25809][table] Improve readability of TableTestPrograms

[twalthr]

What is the purpose of the change

From first feedback, this PR improves the readability of TableTestPrograms.

Brief change log

• Use a proper builder pattern for SourceTestStep and SinkTestStep
• Give builder methods a better name (indicate when it add or sets, indicate whether data is produced or consumed)

Verifying this change

This change is already covered by existing tests.

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? JavaDocs

[flinkbot]

CI report:

• b5c3857 Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

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

[dawidwys]

LGTM