Add unit test for open search client part 1

[neil-xie]

What changed?
Add unit test for open search client client, more to add
Simplified a test in ES client v6 and v7

Why?
Increase test coverage

How did you test it?
Unit test

Potential risks

Release notes

Documentation Changes

[taylanisikdemir]

thanks for simplifying this comparison

[codecov]

Codecov Report

Merging #5774 (b0fca28) into master (ce25878) will decrease coverage by 0.12%.
The diff coverage is n/a.

Additional details and impacted files

see 7 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 018e2e98-332e-4440-bb83-3746605dfe34

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 107 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.003%) to 64.082%

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Files with Coverage Reduction	New Missed Lines	%
service/history/task/timer_task_executor_base.go	2	78.38%
service/history/task/transfer_active_task_executor.go	2	72.51%
common/util.go	2	91.69%
service/matching/taskListManager.go	2	80.96%
common/task/fifo_task_scheduler.go	3	82.47%
common/persistence/statsComputer.go	3	94.64%
common/persistence/nosql/nosqlplugin/cassandra/tasks.go	4	76.09%
service/frontend/wrappers/metered/metered.go	4	65.5%
common/persistence/nosql/nosql_task_store.go	5	61.95%
common/persistence/nosql/nosqlplugin/cassandra/gocql/public/client.go	12	20.93%

Totals
Change from base Build 018e2e69-7db6-4e96-a90a-883272aa1a9f:	-0.003%
Covered Lines:	93420
Relevant Lines:	145782

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💛 - Coveralls