[tests] workaround for relative dataset path
`datasets==2.3.1` introduced a bug where it fails to load a dataset via a path that contains `..`. 

This PR works around it by avoiding this situation in the tests.

@sgugger, @ydshieh 
_The documentation is not available anymore as the PR was closed or merged._<|||||>the just released `datasets==2.3.2` fixed the bug, so closing this PR.