use the "test" scope for the io.grpc:grpc-testing dependency

[votez]

reduces runtime jar and removes unused classes from the distribution.
I.e. JUnit is not needed in prod

[coveralls]

Pull Request Test Coverage Report for Build 2193

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.02%) to 60.244%

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Files with Coverage Reduction	New Missed Lines	%
src/main/java/com/uber/cadence/internal/sync/WorkflowThreadImpl.java	1	77.2%
src/main/java/com/uber/cadence/internal/replay/ReplayDecisionTaskHandler.java	1	87.8%
src/main/java/com/uber/cadence/internal/replay/ReplayDecider.java	5	80.27%

Totals
Change from base Build 2191:	0.02%
Covered Lines:	11344
Relevant Lines:	18830

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💛 - Coveralls

[natemort]

LGTM, rebased.