want examples of unit tests

**Is your feature request related to a problem? Please describe.**

I'm writing an application that using Dapr Service-to-service invocation. I want to write unit tests for service hander, but I can't find any document to do that.

**Describe the solution you'd like**

There is a example will be helpful, maybe like
https://github.com/dapr/go-sdk/blob/main/examples/service/serving/grpc/main_test.go for https://github.com/dapr/go-sdk/blob/main/examples/service/serving/grpc/main.go

Start by comparing the referenced examples/service/serving/grpc/main.go and main_test.go in dapr/go-sdk, then inspect the corresponding service-to-service invocation example in this repository. Add a clearly placed unit-test example for the service handler and verify that it demonstrates the requested testing approach.

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