Fix tests involving temp directory on macOS

[bmuskalla]

Tests that use a mixture SoftTemporaryDirectory and the fx_cache_dir fixture may fail on Mac OS.

The failure usually says something like

FAILED tests/test_file_download.py::HfHubDownloadToLocalDir::test_with_local_dir_and_symlinks_and_overwrite - OSError: [Errno 30] Read-only file system: '/tmpgucu6pwx'

The problem is that the code trying to create a symlink in _create_symlink tries to find a relative path from src to dst in

relative_src = os.path.relpath(abs_src, abs_dst_folder)

The problem is that on Mac OS, abs_src and abs_dst have a different root path due to the way the OS handles temp directories.
The one from the fixture is a resolved path to /private/var/folders/… while the one from SoftTemporaryDirectory is still an unresolved path pointing to /var/folders/ (/var/ is actually var -> private/var). And thus, their common ancestor is / which we try use to to write the symlink to (which obviously fails).

I removed resolving the path from the fixture and instead resolve the path all the time in SoftTemporaryDirectory (which is used by the fixture) so all call sites get the same resolved path.

[HuggingFaceDocBuilderDev]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.63%. Comparing base (c32602a) to head (9425c90).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2052      +/-   ##
==========================================
- Coverage   80.64%   80.63%   -0.02%     
==========================================
  Files          71       71              
  Lines        8519     8519              
==========================================
- Hits         6870     6869       -1     
- Misses       1649     1650       +1     

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[Wauplin]

Thanks for digging into it and fixing this! I like the suggested solution like this :)
As you may have noticed we sometimes have a mix of different solutions, mostly for legacy reasons. We tend to reduce those inconsistencies but often the time + maintenance it takes to update these is just higher than the effective benefit / impact, so that's why it is how it is ^^ Fixing tests is beneficial though! 😄

[Wauplin]

It's usually dangerous to update the return value of an helper but in this case it looks fine 👍

(+1 on using Path as much as possible. Codebase is currently a mix of str and Path for legacy reasons but 🤷)