Change path to posix (testing_utils.py)

[stephen-iezzi]

What does this PR do?

Introduces Posix to provide stronger support for Windows compatibility. Changes the instance of os.path.join() to Path(...).as_posix().

Part of #6585

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Who can review?

@sayakpaul

[sayakpaul]

Thank you!

[sayakpaul]

Thank you. Please ping me when the CI has completed running. I will merge.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[stephen-iezzi]

Glad to help! @sayakpaul CI is finished. 1 check failed, but it's just because of a dependency issue which is addressed in #6800.

[sayakpaul]

Oops we need to run the CI one more time to mitigate the dependency bug.