Expose test client for end-user unit testing

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

I'm writing an application that uses a Dapr state store. I'd like to be able to run unit tests in isolation from external services, so I'd like to use a fake Dapr client in test.

**Describe the solution you'd like**

It looks like this SDK's own unit tests build a private test Dapr client, it would be helpful if that was exposed publicly.

**Describe alternatives you've considered**

I can probably implement my own fake Dapr client, but it would be convenient and more forward-compatible if this was provided by the project itself