Unit Test for ActivityCompletionClient methods that don't use taskToken

[Spikhalskiy]

All the methods of ActivityCompletionClient that don't use task token and use String workflowId, Optional<String> runId, String activityId instead are unused and untested. The same is ManualActivityCompletionClientImpl codepaths that work when workflowId + activityId are specified instead of taskToken.

These methods should be covered with basic unit tests.