Add unit tests for LoadWorkflowExecution method

[Shaddoll]

What changed?
Add unit tests for LoadWorkflowExecution method

Why?
improve coverage

How did you test it?
unit tests

Potential risks

Release notes

Documentation Changes

[coveralls]

Pull Request Test Coverage Report for Build 018ebed1-ed2e-4e89-be5d-2ed9388c9039

Details

• 22 of 24 (91.67%) changed or added relevant lines in 1 file are covered.
• 67 unchanged lines in 15 files lost coverage.
• Overall coverage decreased (-0.001%) to 67.272%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
service/history/execution/context.go	22	24	91.67%

Files with Coverage Reduction	New Missed Lines	%
service/history/task/transfer_standby_task_executor.go	2	87.42%
common/persistence/execution_manager.go	2	83.54%
common/task/fifo_task_scheduler.go	2	83.51%
common/persistence/sql/sqlplugin/mysql/task.go	2	73.68%
service/matching/db.go	2	73.23%
service/history/task/transfer_active_task_executor.go	2	72.83%
service/history/execution/mutable_state_builder.go	2	70.28%
common/persistence/statsComputer.go	2	96.07%
common/persistence/sql/sqlplugin/mysql/db.go	2	79.49%
common/log/tag/tags.go	3	50.74%

Totals
Change from base Build 018ebec1-be6e-4fd2-8fe7-d5142323d1df:	-0.001%
Covered Lines:	98053
Relevant Lines:	145755

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💛 - Coveralls

[codecov]

Codecov Report

Merging #5880 (b913814) into master (88a4dea) will increase coverage by 0.06%.
Report is 1 commits behind head on master.
The diff coverage is 47.82%.

❗ Current head b913814 differs from pull request most recent head 22eaaf4. Consider uploading reports for the commit 22eaaf4 to get more accurate results

Additional details and impacted files

Files	Coverage Δ
service/history/execution/context.go	81.84% <47.82%> (+8.42%)	⬆️

... and 7 files with indirect coverage changes

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