Add unimplemented interop test

tonic-interop/src/server.rs

@@ -152,6 +152,6 @@ impl pb::server::TestService for TestService {
     }
 
     async fn unimplemented_call(&self, _: Request<Empty>) -> Result<Empty> {
-        unimplemented!()
+        Err(Status::unimplemented(""))
     }
 }

tonic-interop/test.sh

@@ -45,6 +45,6 @@ trap 'echo ":; killing test server"; kill ${SERVER_PID};' EXIT
 
 ./target/debug/client \
 --test_case=empty_unary,large_unary,client_streaming,server_streaming,ping_pong,\
-empty_stream,status_code_and_message,special_status_message $ARG
-# unimplemented_method,unimplemented_service,custom_metadata
+empty_stream,status_code_and_message,special_status_message,unimplemented_method $ARG
+# ,unimplemented_service,custom_metadata