Fix flaky tests in CopyProcessorTests

[gaobinlong]

Description

When testing CopyProcessorTests#testCopyWithRemoveSource(), even though we use random() to generate field names, but there's a probability that the target field's name conflicts with the existing fields(then the error target field [] already exists will be thrown), we need to have a better way to make sure that the target field's name never exists in the document.

Another method testCopyExistingField() also has the same issue.

Related Issues

#12884

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Failing checks are inspected and point to the corresponding known issue(s) (See: Troubleshooting Failing Builds)
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)
• Public documentation issue/PR created

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[github-actions]

Compatibility status:

Checks if related components are compatible with change 0d79235

Incompatible components

Skipped components

Compatible components

Compatible components: [https://github.com/opensearch-project/custom-codecs.git, https://github.com/opensearch-project/asynchronous-search.git, https://github.com/opensearch-project/observability.git, https://github.com/opensearch-project/reporting.git, https://github.com/opensearch-project/flow-framework.git, https://github.com/opensearch-project/job-scheduler.git, https://github.com/opensearch-project/cross-cluster-replication.git, https://github.com/opensearch-project/opensearch-oci-object-storage.git, https://github.com/opensearch-project/common-utils.git, https://github.com/opensearch-project/geospatial.git, https://github.com/opensearch-project/k-nn.git, https://github.com/opensearch-project/neural-search.git, https://github.com/opensearch-project/anomaly-detection.git, https://github.com/opensearch-project/security-analytics.git, https://github.com/opensearch-project/sql.git, https://github.com/opensearch-project/notifications.git, https://github.com/opensearch-project/ml-commons.git, https://github.com/opensearch-project/index-management.git, https://github.com/opensearch-project/alerting.git, https://github.com/opensearch-project/performance-analyzer-rca.git, https://github.com/opensearch-project/security.git, https://github.com/opensearch-project/performance-analyzer.git]

[github-actions]

❕ Gradle check result for 8e87efb: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.remotestore.RemoteStoreStatsIT.testNonZeroPrimaryStatsOnNewlyCreatedIndexWithZeroDocs

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.34%. Comparing base (b15cb0c) to head (2c039d7).
Report is 93 commits behind head on main.

❗ Current head 2c039d7 differs from pull request most recent head 0d79235. Consider uploading reports for the commit 0d79235 to get more accurate results

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #12885      +/-   ##
============================================
- Coverage     71.42%   71.34%   -0.08%     
- Complexity    59978    60254     +276     
============================================
  Files          4985     5011      +26     
  Lines        282275   283675    +1400     
  Branches      40946    41117     +171     
============================================
+ Hits         201603   202399     +796     
- Misses        63999    64554     +555     
- Partials      16673    16722      +49     

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[github-actions]

❌ Gradle check result for 955a84a: FAILURE

Please examine the workflow log, locate, and copy-paste the failure(s) below, then iterate to green. Is the failure a flaky test unrelated to your change?

[github-actions]

✅ Gradle check result for 2c039d7: SUCCESS

[gaobinlong]

@reta ,could you help to review this PR, thank you!

[github-actions]

❕ Gradle check result for 0d79235: UNSTABLE

• TEST FAILURES:

      1 org.opensearch.action.admin.cluster.node.tasks.ResourceAwareTasksTests.testTaskResourceTrackingDuringTaskCancellation

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.