[FLINK-12313] Add workaround to avoid race condition in SynchronousCheckpointITCase test

[1u0]

What is the purpose of the change

The SynchronousCheckpointITCase has a race condition and with some chance may fail on random tests on CI. This PR adds an additional synchronization point as a workaround to avoid the issue.

Additionally, the PR simplifies (refactors) the test, although the race condition is present in both refactored and non-refactored versions.

Brief change log

• The test is simplified to not use dedicated locks for general flow checks. The test run set to limited by a timeout.
• Added a hacky synchronization point in advanceToEndOfEventTime method, to make sure that triggerCheckpoint thread has progressed far enough, before triggering notifyCheckpointComplete.

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, 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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• ❓ 1. The [description] looks good.
• ❓ 2. There is [consensus] that the contribution should go into to Flink.
• ❗ 3. Needs [attention] from.
  • Needs attention by @StefanRRichter [committer], @pnowojski [committer]
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• ❓ 5. Overall code [quality] is good.

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[1u0]

@flinkbot attention @pnowojski @StefanRRichter

[pnowojski]

I've left couple of questions.

[pnowojski]

LGTM % timeout. If you don't want to change it, let me know and I will merge it, but in that case please monitor jira issues if someone is reporting this test as flaky.

[1u0]

@pnowojski, thank you for the review and feedback.

I've updated the test timeout to 10 seconds, and propose to stick with it for now.
If it would fail in the future due to too small time window and if I notice it, I can open a new PR to increase it.

[pnowojski]

I'll merge it once travis is green (previous build has failed on some unrelated issue).

[pnowojski]

Travis is green, merging.