There's a set of interop tests defined in:
https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md
If you implement the client/server, you can run the tests as Julia to Julia, or between Julia and other languages.
Some basic tests to start with are: empty_unary, client_streaming, server_streaming, ping_pong.
And there are also tests for more features, like custom_metadata, status_code_and_message, cancellation, timeout.