Test/Mock Agora Server Not Exported

[zfedoran]

There is a nice mock server service available in the test_server.go file but unfortunately it is not exported (capitalized). It is used to validate the client logic internally.

Anyone using the kin-go sdk will likely want to write test cases of their own and in order to do that, it would be nice if the existing test_server.go file exported both the newServer() function and server type by uppercasing them.