Improve CustomizedStateProvider tests

[mgao0]

Issues

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

#728

Description

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This PR added tests to make CustomizedStateProvider tests comprehensive.

Tests

• The following tests are written for this issue:

TestCustomizedStateUpdate

• The following is the result of the "mvn test" command on the appropriate module:

START TestCustomizedStateUpdate at Mon Mar 02 17:34:44 PST 2020
START testDeleteNonExistingPerPartitionCustomizedState at Mon Mar 02 17:34:46 PST 2020

Default Suite
Total tests run: 6, Failures: 0, Skips: 0

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  4. Subject uses the imperative mood ("add", not "adding")
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  6. Body explains "what" and "why", not "how"

Documentation (Optional)

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Code Quality

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[zhangmeng916]

Great work in adding these test cases!

[mgao0]

This PR is ready to be merged, approved by @zhangmeng916 .