Refactor conftest fixtures

[albertvillanova]

Previously, fixture modules hub_fixtures and s3_fixtures:

• were both at the root test directory
• were imported using import *
  • as a side effect, the modules os and pytest were imported from s3_fixtures into conftest

This PR:

• puts both fixture modules in a dedicated directory fixtures
• renames both to: fixtures.hub and fixtures.s3
• imports them into conftest as plugins, using the pytest_plugins: this avoids the import *
• additionally creates a new fixture module fixtures.files with all file-related fixtures

[HuggingFaceDocBuilderDev]

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