[tests] relocate core integration tests
This PR 

* moves `deepspeed`/`fairscale`/extended trainer tests from `examples` to `tests`
* updates docs to point to the new sample config files
* adds a new `testing_utils.py`'s context manager `ExtendSysPath` that allows temporary `sys.path` changing to import something locally in the tests and uses it (otherwise sagemaker tests were breaking because they contain `__init__.py`) + doc

Hopefully, it'll be the new home for integration tests for awhile, - specifically for deepspeed tests as the DeepSpeed team would like to run our tests as part of their CIs.

We still need to split off the `fairscale` tests once we start working on this integration again, so for now just moving as is.

@sgugger 
@sgugger, @LysandreJik - so after this move we have a problem of dependencies now - the extended integration tests need be able to check score metrics, but the main tests don't have `sacrebleu` and other dependencies installed. How do we resolve this conundrum?

Lysandre replied on slack to add them to `extras["testing"]` - so doing that.<|||||>I think we can add
```
sacrebleu >= 1.4.12
rouge-score
nltk
```
to the testing extra. It should be all you need.