[CI/Build] Update pixtral tests to use JSON

[DarkLight1337]

I can verify that the Pixtral tests after #8415 pass on my L40. However having pickle files in the repo may be flagged as a security risk, so I'm changing them to JSON in this PR.

cc @patrickvonplaten

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[ywang96]

Do you want to remove the skip decorator and see if it actually passes on our CI?

[DarkLight1337]

Even the smallest test takes up 40 GB VRAM so it will cause OOM in CI.

[DarkLight1337]

We probably have to split the model using TP if we want to run the tests in CI.

[ywang96]

Let’s skip the TP addition for a later PR then

[patrickvonplaten]

Thanks a lot @DarkLight1337 - make sense pickle is not the best format