Add unit tests for methods in frontend api handler

[timl3136]

What changed? Add unit tests for StartWorkflowExecution and SignalWorkflowExecution methods in frontend api handler

Why? Improve test coverage

How did you test it? Unit tests

Potential risks

Release notes

Documentation Changes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.09%. Comparing base (8ad179a) to head (be26b46). Report is 2 commits behind head on master.

Additional details and impacted files

[see 12 files with indirect coverage changes](https://app.codecov.io/gh/uber/cadence/pull/6123/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber) ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/uber/cadence/pull/6123?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/uber/cadence/pull/6123?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber). Last update [8ad179a...be26b46](https://app.codecov.io/gh/uber/cadence/pull/6123?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber).

[coveralls]

Pull Request Test Coverage Report for Build 018fef9a-2a96-4c4f-9126-93535752ef5e

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 70 unchanged lines in 17 files lost coverage.
• Overall coverage decreased (-0.009%) to 71.393%

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Files with Coverage Reduction	New Missed Lines	%
service/history/queue/timer_queue_processor_base.go	1	77.66%
common/task/parallel_task_processor.go	2	93.06%
service/matching/tasklist/task_list_manager.go	2	77.05%
service/history/task/transfer_active_task_executor.go	2	72.77%
common/util.go	2	91.84%
service/history/execution/mutable_state_builder.go	2	78.45%
common/membership/hashring.go	2	84.69%
service/history/task/fetcher.go	2	85.05%
common/persistence/historyManager.go	2	66.67%
service/history/task/transfer_standby_task_executor.go	3	87.55%
<!--	Total:	70		-->

Totals
Change from base Build 018fef90-a615-49da-9a34-4252dfbfb34c:	-0.009%
Covered Lines:	106345
Relevant Lines:	148957

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💛 - Coveralls