Add tests for elasticsearch v6 client

[neil-xie]

What changed?
Add unit test for NewV6Client, CreateIndex, PutMapping and Count

Why?
Improve test coverage

How did you test it?
Unit test

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 018e1071-f41c-4de3-a965-1763d1ef5be4

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 49 unchanged lines in 13 files lost coverage.
• Overall coverage increased (+0.05%) to 63.2%

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Files with Coverage Reduction	New Missed Lines	%
common/task/weighted_round_robin_task_scheduler.go	1	89.05%
service/history/queue/timer_queue_processor_base.go	1	77.62%
service/matching/taskGC.go	1	97.37%
common/membership/hashring.go	2	82.23%
common/persistence/historyManager.go	2	66.67%
common/persistence/sql/sqlplugin/postgres/db.go	2	80.0%
common/persistence/sql/sqlplugin/postgres/task.go	2	73.4%
service/history/execution/mutable_state_builder.go	2	68.74%
service/history/task/transfer_active_task_executor.go	2	72.38%
service/matching/matcher.go	2	91.41%

Totals
Change from base Build 018e1070-ebcc-4449-b3da-fbe3488a2176:	0.05%
Covered Lines:	92693
Relevant Lines:	146665

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💛 - Coveralls

[codecov]

Codecov Report

Merging #5716 (ef45eab) into master (bed1969) will decrease coverage by 0.08%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

❗ Current head ef45eab differs from pull request most recent head 035a4b4. Consider uploading reports for the commit 035a4b4 to get more accurate results

Additional details and impacted files

see 8 files with indirect coverage changes

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