fix TestCleanupExternalView.test

[kaisun2000]

Issues

• My PR addresses the following Helix issues and references them in the PR description:

fix #1458

Description

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  Flaky test with 10 * 100ms. This may not be enough when there is
  logging or Zk is slow in github. Fix with TestHelper.verify();

Tests

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github run

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Commits

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  4. Subject uses the imperative mood ("add", not "adding")
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Code Quality

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[zhangmeng916]

LGTM

[kaisun2000]

This diff is approved. Please help to merge

Flaky test with 10 * 100ms. This may not be enough when there is
logging or Zk is slow in github. Fix with TestHelper.verify();