Fix test assertion in test_trainer.py

[slowder]

Fix: replace incorrect call to assertTrue with assertEqual

What does this PR do?

As originally written, the call to assertTrue simply asserts that the len(preds) is a truthy value rather than comparing to the second argument.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[jimypbr]

LGTM thanks!

[HuggingFaceDocBuilderDev]

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