[FLINK-26993][tests] Wait until checkpoint was actually triggered

[akalash]

What is the purpose of the change

Backport to release-1.15 of #19356

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, 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)

[flinkbot]

CI report:

• cdc7a37 Azure: SUCCESS

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[pnowojski]

Change LGTM, but I couldn't figure out what has happened in this test failure. I can not figure out which test has failed, am I missing something obvious? I see this being logged:

Jun 01 22:02:55 [ERROR] pure virtual method called
Jun 01 22:02:55 [ERROR] terminate called without an active exception
Jun 01 22:02:59 [ERROR] Aborted (core dumped)

[akalash]

@pnowojski ,You can check logs from this ticket for example - https://issues.apache.org/jira/browse/FLINK-27216(it is a fresh duplicate of FLINK-26993). There is a pretty clear NPE exception(https://dev.azure.com/apache-flink/apache-flink/_build/results?buildId=34535&view=logs&j=0da23115-68bb-5dcd-192c-bd4c8adebde1&t=24c3384f-1bcb-57b3-224f-51bf973bbee8&l=9180) which happens because we call receiveAcknowledgeMessage before the PendingCheckpoint was initialized(it happens only in test since we trigger it manually but it is an impossible situation in reality). It was broken when we separate setting checkpoint storage from creating of the PendingCheckpoint.

[akalash]

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