Added mocks for interfaces and also unit tests for functions used in history handler

[timl3136]

What changed?
Added mocks for interface and also unit tests for functions used in history handler

Why?
Improve test coverage and stability

How did you test it?
Unit tests

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

Merging #5882 (b241c0b) into master (3479700) will decrease coverage by 0.41%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

❗ Current head b241c0b differs from pull request most recent head 7bf151d. Consider uploading reports for the commit 7bf151d to get more accurate results

Additional details and impacted files

Files	Coverage Δ
common/taskTokenSerializerInterfaces.go	0.00% <ø> (ø)
service/history/events/notifier.go	78.94% <ø> (ø)

... and 9 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 018eb0a5-c789-4a11-9244-a5679bd84309

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 46 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.03%) to 67.227%

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Files with Coverage Reduction	New Missed Lines	%
service/matching/taskReader.go	2	84.88%
service/history/execution/mutable_state_task_refresher.go	2	62.03%
service/history/queue/timer_gate.go	3	95.83%
service/history/task/fetcher.go	5	85.57%
service/history/task/transfer_standby_task_executor.go	6	86.19%
service/history/task/cross_cluster_task_processor.go	8	80.79%
service/history/task/task_util.go	20	70.57%

Totals
Change from base Build 018eb09a-ebf9-4f53-a677-082ee6ceb39f:	0.03%
Covered Lines:	98137
Relevant Lines:	145979

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💛 - Coveralls