[Tests] add a test suite to check if loading from subfolder works as expected.

[sayakpaul]

What does this PR do?

To prevent things like this from happening: https://github.com/huggingface/diffusers/pull/8416

[sayakpaul]

@yiyixuxu do the recent changes work for you?

(Have tested them too)

Regardless, I think having a separate suite for subfolder tests wouldn't be too bad.

[sayakpaul]

@yiyixuxu a gentle ping here.

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