Refacto Repository tests

[Wauplin]

TL;DR:

1. Sorry for huge diff 😢
2. Repository tests work on Windows (related to Add windows machine for CI #1112)
3. Repository tests run in parallel
4. Repository tests use /tmp dir
5. Less redundancy

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First of all, sorry for the huge diff in the PR 😢 .
I started it as a preliminary work for #1112 (Windows CI):

1. I started to refacto how paths are generated => no more "directory/whatever_file.txt" as it breaks on Windows
2. While doing that, I removed the WORKING_DIR_FIXTURE to replace it by a proper /tmp folder
   => Avoid to have to clean it after tests (when tests were failing, I often ended up with untracked folders in my huggingface_hub folder that I had to delete)
3. Then I read more the tests and realized we were doing a lot of redundant work (creating 1 new repo on the Hub for each test even though we do not modify it)
   1. I've split repository tests between TestRepositoryShared and TestRepositoryUniqueRepos. In shared test, a single repo is created on the Hub and cloned multiple times. No files are pushed to this repo during the tests.
      => Save quite some time of setup/teardown. In Unique test, a repo is created per test (same as before) so that they are independent from each other.
   2. I also added attributes like self.repo_id, self.repo_path and self.repo_url and an helper self.clone_from(...) to avoid reduncancy (e.g. recomputing repo_id=f"{USER}/{self.REPO_NAME}", local_dir="{WORKING_DIR}/{REPO_NAME}" all the time).
4. And finally I removed some tests that were duplicated (*).

=> Now we have a Repository test suite that should be "iso" compared to before but running in parallel (~1min instead of 8) and on Windows.

(*) For example in dataset tests, we are testing that cloning from a different repo type works. But also testing that we can commit, pull, push,... which is quite redundant because what we really want is to check if a repo can be cloned with a repo_type. All subsequent actions are already tested separately ("test only 1 feature at a time").

(EDIT: "non repository" tests are failing but it's not due to this PR 😕)

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[LysandreJik]

Much cleaner this way! Thanks for the refactor @Wauplin.

[Wauplin]

Thanks @LysandreJik for the review and sorry again for the big diff to review. I'll fix conflicts, wait for the checks and merge it :)