Add unit tests to ActivityCompletionException

[timl3136]

What changed?
Add unit tests to ActivityCompletionException

Why?
Improve code coverage

How did you test it?
Unit test

Potential risks

Release notes

Documentation Changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.61%. Comparing base (aa86d2a) to head (4be43e4).
Report is 9 commits behind head on master.

Additional details and impacted files

see 11 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 2626

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 292 unchanged lines in 6 files lost coverage.
• Overall coverage decreased (-1.3%) to 78.233%

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Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/uber/cadence/internal/sync/CancellationScopeImpl.java	1	92.65%
src/main/java/com/uber/cadence/internal/sync/TestWorkflowEnvironmentInternal.java	21	54.14%
src/main/java/com/uber/cadence/internal/replay/SignalExternalWorkflowParameters.java	22	42.11%
src/main/java/com/uber/cadence/internal/compatibility/thrift/ErrorMapper.java	30	0.0%
src/main/java/com/uber/cadence/internal/testservice/TestWorkflowService.java	44	72.66%
src/main/java/com/uber/cadence/internal/logging/ReplayAwareLogger.java	174	6.95%

Totals
Change from base Build 2625:	-1.3%
Covered Lines:	15167
Relevant Lines:	19387

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💛 - Coveralls