[FLINK-17344] Fix unstability of RecordWriterTest. testIdleTime and TaskMailboxProcessorTest.testIdleTime

[wenlong88]

What is the purpose of the change

Fix unstability on RecordWriterTest. testIdleTime and TaskMailboxProcessorTest.testIdleTime

Brief change log

Add an CountDownLatch to avoid race condition between threads.

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/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)

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Automated Checks

Last check on commit b6a842d (Fri Apr 24 02:24:02 UTC 2020)

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[pnowojski]

LGTM.

Just small issue for the future, please include some explanation in the PR description and in the commit message like:

Previously issue was that a thread could finish before the main thread reached .wait() call

[pnowojski]

Merged as 948a42f