Add CallHelpers reference in test-client documentation (#36531)

Author: kzu

aspnetcore/grpc/test-client.md

@@ -48,6 +48,7 @@ The preceding unit test:
 * Mocks `IGreetRepository` and `TesterClient` using [Moq](https://www.nuget.org/packages/Moq).
 * Starts the worker.
 * Verifies `SaveGreeting` is called with the greeting message returned by the mocked `TesterClient`.
+* Uses the [`CallHelpers` from the gRPC examples](https://github.com/grpc/grpc-dotnet/blob/2e283d98f5861e31f8956be0c68aff9a4e29cb0a/examples/Tester/Tests/Client/Helpers/CallHelpers.cs)
 
 ## Additional resources