CI: fix test-save-trainer

[zucchini-nlp]

What does this PR do?

As per title, fixes the test. The default directory is indeed called trainer_output, so the test is not correctly checking it

[SunMarc]

Thanks ! Indeed, we changed that recently.

[ydshieh]

"""Test that output_dir defaults to 'tmp_trainer' when not specified."""

this has to be adjusted too.

Thank you

[HuggingFaceDocBuilderDev]

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