[examples] fix sys.path in conftest.py
the pt/tf reshuffle broke the examples tests setup. This PR fixes it, by 

* fixing `sys.path` setting now that it's one level down 

@sgugger 
That can't work since `conftest` now believes the module "tensorflow" is the folder in the examples with the same name, which is the reason why I moved it.

Path in conftest need to be adapted instead.<|||||>ah, OK, then I will make copies of it. But you can't move it since sys.path was relying on it being in a top-level subfolder - so will have to fix that or make the code more flexible. It's currently broken.

I will take care of it.

thank you for letting me know tf didn't like it.<|||||>ok, 

1. put `conftest.py` back and fixed it to set up `sys.path` correctly
2. made 2 copies of it for legacy/research.<|||||>> The fix in itself LGTM but why add a conftest to legacy and research-projects? Both are not supposed to be tested.

I'm adding deepspeed tests for wav2vec2 - it's been complicated to make it work (some parts still don't work). So I want to make sure it doesn't break again.

I don't think it's a good idea to put these tests under common tests.

So I can't run the tests w/o this conftest on dev box.<|||||>I really don't think it's a good idea to add this to `research-projects`: all research projects have a pinned version of Transformers and this would use the master install to test them, which is incompatible with what is advertised.

Maybe add it specifically to the wav2vec2 research project? Or maybe wav2vec2 should move to a maintained example @patrickvonplaten ?<|||||>> I really don't think it's a good idea to add this to `research-projects`: all research projects have a pinned version of Transformers and this would use the master install to test them, which is incompatible with what is advertised.

Good point! 

OK, will recall the 2 other files for now.

> Maybe add it specifically to the wav2vec2 research project? Or maybe wav2vec2 should move to a maintained example @patrickvonplaten ?

Yes, totally agree!<|||||>`pathlib` is easier to use for finding out older ancestors than doing `dirname` 4 times:
```
import sys
from pathlib import Path
git_repo_path = Path(__file__).resolve().parents[3] / "src"
sys.path.insert(1, str(git_repo_path))
```