Add unit tests for WorkflowExecutionUtils#getInstanceCloseEvent

[Spikhalskiy]

WorkflowExecutionUtils right now is completely untestable and unmockable, we should decouple this class into testable pieces to make it possible to cover it with unit tests.
The first unit test worth adding is a unit test for an issue fixed in #472

[Spikhalskiy]

This was mostly covered by extensive GrpcRetryer reworking and testing done in the last couple of months.