Add unit test for util methods in es v6 client bulk processor

[neil-xie]

What changed?
Add unit test for util methods in es v6 client bulk processor
Fix a typo in fromV6ToGenericBulkResponse method name

Why?
Improve test coverage

How did you test it?
Unit test

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

Merging #5748 (a57945d) into master (bc5cc93) will increase coverage by 0.03%.
The diff coverage is 50.00%.

❗ Current head a57945d differs from pull request most recent head 2dd7c76. Consider uploading reports for the commit 2dd7c76 to get more accurate results

Additional details and impacted files

Files	Coverage Δ
common/elasticsearch/client/v6/client_bulk.go	37.16% <50.00%> (+37.16%)	⬆️

... and 6 files with indirect coverage changes

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