Rm extracted test dirs
Thanks for cleaning up the extracted dummy data folders! Instead of changing the file_utils we could also just put these folders under `.gitignore` (or maybe already done?). Awesome! I guess you might have to add the changes for the MockDLManager now in a different file though because of my last PR - sorry!
All the dummy data used for tests were duplicated. For each dataset, we had one zip file but also its extracted directory. I removed all these directories

Furthermore instead of extracting next to the dummy_data.zip file, we extract in the temp `cached_dir` used for tests, so that all the extracted directories get removed after testing.

Finally there was a bug in the `mock_download_manager` that would let it create directories with invalid names, as in #172. I fixed that by encoding url arguments. I had to rename the dummy data for `scientific_papers` and `cnn_dailymail` (the aws tests don't pass for those 2 in this PR, but they will once aws will be synced, as the local ones do)

Let me know if it sounds good to you @patrickvonplaten . I'm still not entirely familiar with the mock downloader