added json dump and extraction of train run time
# What does this PR do?

This PR adjusts to the latest metric exposing changes that `train_runtime` is now logged as `hh:mm:ss.ms`. So instead of extracting the `train_runtime` from the logs it is using the `sagemaker-sdk` to get the full train time. Additionally, I added a JSON dump for all tests to share the result easier, when opening a new PR to upgrade the HF DLC. 