[FLINK-22338][table-planner] Clear watermark output when test finished for FromElementSourceFunctionWithWatermark

[fsk119]

…d for FromElementSourceFunctionWithWatermark

What is the purpose of the change

Clear the watermark output per test. To reproduce this bug, please run SourceWatermarkITCase twice.

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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Automated Checks

Last check on commit ece93ac (Sat Aug 28 11:14:34 UTC 2021)

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• ❓ 1. The [description] looks good.
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• ❓ 5. Overall code [quality] is good.

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CI report:

• ece93ac Azure: FAILURE

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[JingsongLi]

Thanks @fsk119 , Looks good to me.