[testing] distributed: correct subprocess output checking
This PR fixes an issue revealed on CI - https://github.com/huggingface/transformers/runs/1327577422

* the external subprocess runner will now be more flexible and check `stdout|stderr` to validate that the subprocess sent at least some output. Currently the code checks only `stdout` which isn't right since the subprocess may not generate any.
* adds `stdout:` prefix to subprocess' stdout, like it was already doing for `stderr`.

@sgugger 