[FLINK-37194][tests] Fix NPE issue in WatermarkITCase

[showuon]

What is the purpose of the change

Fix NPE issue in WatermarkITCase.
The NPE is because here, the latter operation (i.e. receivedWatermarks.computeIfAbsent) might not be run immediately after the former operation (i.e. attemptIds.compute).

And during the main testing thread, we only wait for attemptIds size is expected. So, after the attemptIds size is expected, when we try to verify assertOperatorReceivedWatermarkValues, we might get null value because in the other thread, the receivedWatermarks.computeIfAbsent is not executed, yet.

To fix this issue, we just need to wait for the receivedWatermarks size, not the attemptIds size. But since this issue exists in many other test cases in WatermarkITCase, I extracted the common codes into a helper method: submitJobAndWaitForOperator3Initialized.

Brief change log

Fix NPE issue in WatermarkITCase.

Verifying this change

This change is already covered by existing tests, such as (please describe 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? not applicable

[showuon]

Here is the main fix, by waiting for receivedWatermarks size, not attemptIds size.

[codenohup]

LGTM, Thanks for the fix.

[flinkbot]

CI report:

• c227934 Azure: PENDING

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