Fix `mlflow can serve a model function test` test in R (#7072)
Fix `mlflow can serve a model function test` test in R (#7072)

* Fix mlflow can serve a model function test

Signed-off-by: harupy <e7c128a96f7b207246da7c828c67136267e8d868@gmail.com>

* check whether server is alive

Signed-off-by: harupy <e7c128a96f7b207246da7c828c67136267e8d868@gmail.com>

* Write to stderr

Signed-off-by: harupy <e7c128a96f7b207246da7c828c67136267e8d868@gmail.com>

* Use wait_for_server_to_start

Signed-off-by: harupy <e7c128a96f7b207246da7c828c67136267e8d868@gmail.com>

Signed-off-by: harupy <e7c128a96f7b207246da7c828c67136267e8d868@gmail.com>
mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow