Add unit tests for methods in frontend api handler

[timl3136]

What changed? Add unit tests for RespondActivityTaskCanceled and missing lines in RespondActivityTaskFailedByID

Why? Improve test coverage

How did you test it? Unit tests

Potential risks

Release notes

Documentation Changes

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.15%. Comparing base (ab0d875) to head (467bef6). Report is 10 commits behind head on master.

:exclamation: Current head 467bef6 differs from pull request most recent head df1d0cb

Please upload reports for the commit df1d0cb to get more accurate results.

Additional details and impacted files

| [Files](https://app.codecov.io/gh/uber/cadence/pull/6100?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber) | Coverage Δ | | |---|---|---| | [service/frontend/admin/handler.go](https://app.codecov.io/gh/uber/cadence/pull/6100?src=pr&el=tree&filepath=service%2Ffrontend%2Fadmin%2Fhandler.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber#diff-c2VydmljZS9mcm9udGVuZC9hZG1pbi9oYW5kbGVyLmdv) | `83.19% <100.00%> (+25.63%)` | :arrow_up: | | [common/domain/dlq\_message\_handler.go](https://app.codecov.io/gh/uber/cadence/pull/6100?src=pr&el=tree&filepath=common%2Fdomain%2Fdlq_message_handler.go&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber#diff-Y29tbW9uL2RvbWFpbi9kbHFfbWVzc2FnZV9oYW5kbGVyLmdv) | `94.28% <66.66%> (ø)` | | ... and [19 files with indirect coverage changes](https://app.codecov.io/gh/uber/cadence/pull/6100/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/6100?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/6100?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber). Last update [ab0d875...df1d0cb](https://app.codecov.io/gh/uber/cadence/pull/6100?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).