Refactor Hub tests
_The documentation is not available anymore as the PR was closed or merged._
This PR refactors `test_upstream_hub` by removing unittests and using the following pytest Hub fixtures:
- `ci_hub_config`
- `set_ci_hub_access_token`: to replace setUp/tearDown
- `temporary_repo` context manager: to replace `try... finally`
- `cleanup_repo`: to delete repo accidentally created if one of the tests fails

This is a preliminary work done to manage unit/integration tests separately.