Add more tests to execution manager

[3vilhamster]

What changed?
I've introduced more test to execution manager.

Side note: changed how history event itema re copied to avoid extra allocation.

Why?
Improving unit test coverage

How did you test it?

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 018fce80-b408-43f6-8dda-e79137af6338

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 54 unchanged lines in 15 files lost coverage.
• Overall coverage increased (+0.008%) to 69.644%

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Files with Coverage Reduction	New Missed Lines	%
service/history/queue/timer_queue_processor_base.go	1	78.28%
common/task/weighted_round_robin_task_scheduler.go	2	88.56%
common/persistence/execution_manager.go	2	90.22%
common/task/fifo_task_scheduler.go	2	85.57%
common/util.go	2	91.78%
service/matching/tasklist/task_writer.go	2	82.63%
common/persistence/statsComputer.go	2	97.14%
service/matching/tasklist/task_reader.go	2	77.45%
service/history/task/transfer_standby_task_executor.go	3	87.96%
service/history/task/task.go	3	84.81%

Totals
Change from base Build 018fcd8a-6d92-406f-9861-fe59d0f7e5a7:	0.008%
Covered Lines:	102981
Relevant Lines:	147868

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💛 - Coveralls