Add tests for history decision task handler

[ketsiambaku]

What changed?
The following tests added in service/history/decision:
TestHandleDecisionStartTimer
TestHandleDecisionCompleteWorkflow
TestHandleDecisionFailWorkflow

Why?
improve code coverage in the package

How did you test it?
unit tests

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.57%. Comparing base (fb4bfce) to head (1830694).
Report is 2 commits behind head on master.

Additional details and impacted files

see 10 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 018fa6b3-4bdc-4962-b180-08089f1eee69

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 117 unchanged lines in 12 files lost coverage.
• Overall coverage decreased (-0.008%) to 69.342%

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Files with Coverage Reduction	New Missed Lines	%
service/history/task/transfer_standby_task_executor.go	2	86.21%
service/matching/tasklist/db.go	2	73.23%
service/matching/tasklist/task_list_manager.go	2	76.09%
service/matching/tasklist/task_writer.go	2	82.63%
service/history/task/task.go	3	84.81%
common/log/tag/tags.go	3	50.46%
common/task/fifo_task_scheduler.go	3	84.54%
service/history/task/timer_active_task_executor.go	3	79.33%
common/persistence/nosql/nosql_task_store.go	5	63.97%
service/history/execution/cache.go	6	74.61%

Totals
Change from base Build 018fa5a0-da9b-4f76-9263-3511c2f681f9:	-0.008%
Covered Lines:	102165
Relevant Lines:	147336

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💛 - Coveralls