[FLINK-18369] Fix instable TableEnvironmentITCase#testStatementSetWithSameSinkTableNames

[dawidwys]

What is the purpose of the change

This PR fixes and reenables TableEnvironmentITCase#testStatementSetWithSameSinkTableNames.

Brief change log

See commit message.

Verifying this change

I run the test ~1000 times without a failure

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• 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)

[dawidwys]

cc @godfreyhe Could you have a look at this PR?

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Automated Checks

Last check on commit 2be78fe (Fri Jun 19 11:30:55 UTC 2020)

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[godfreyhe]

@dawidwys thanks for the fix. LGTM